workflows/eval: load supportedSystems from JSON file

This is a refactor to prepare the next commit. It doesn't do much on its
own, but is separated for ease of review.

.github/workflows/eval.yml

···
     if: needs.get-merge-commit.outputs.mergedSha
     outputs:
       targetSha: ${{ steps.targetSha.outputs.targetSha }}
-      systems: ${{ steps.systems.outputs.systems }}
     steps:
       - name: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
···
         with:
           extra_nix_config: sandbox = true
 
-      - name: Evaluate the list of all attributes and get the systems matrix
-        id: systems
+      - name: Evaluate the list of all attributes
         run: |
           nix-build nixpkgs/ci -A eval.attrpathsSuperset
-          echo "systems=$(<result/systems.json)" >> "$GITHUB_OUTPUT"
 
       - name: Upload the list of all attributes
         uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
···
     strategy:
       fail-fast: false
       matrix:
-        system: ${{ fromJSON(needs.attrs.outputs.systems) }}
+        system: ${{ fromJSON(needs.get-merge-commit.outputs.systems) }}
     steps:
       - name: Enable swap
         run: |

.github/workflows/get-merge-commit.yml

···
       mergedSha:
         description: "The merge commit SHA"
         value: ${{ jobs.resolve-merge-commit.outputs.mergedSha }}
+      systems:
+        description: "The supported systems"
+        value: ${{ jobs.resolve-merge-commit.outputs.systems }}
 
 permissions: {}
 
···
     runs-on: ubuntu-24.04-arm
     outputs:
       mergedSha: ${{ steps.merged.outputs.mergedSha }}
+      systems: ${{ steps.systems.outputs.systems }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
···
               fi
               ;;
           esac
-          rm -rf base
+
+      - name: Load supported systems
+        id: systems
+        run: |
+          echo "systems=$(jq -c <base/ci/supportedSystems.json)" >> "$GITHUB_OUTPUT"

ci/eval/README.md

···
   --arg evalSystems '["x86_64-linux" "aarch64-darwin"]'
 ```
 
-- `--max-jobs`: The maximum number of derivations to run at the same time. Only each [supported system](../supportedSystems.nix) gets a separate derivation, so it doesn't make sense to set this higher than that number.
+- `--max-jobs`: The maximum number of derivations to run at the same time. Only each [supported system](../supportedSystems.json) gets a separate derivation, so it doesn't make sense to set this higher than that number.
 - `--cores`: The number of cores to use for each job. Recommended to set this to the amount of cores on your system divided by `--max-jobs`.
 - `chunkSize`: The number of attributes that are evaluated simultaneously on a single core. Lowering this decreases memory usage at the cost of increased evaluation time. If this is too high, there won't be enough chunks to process them in parallel, and will also increase evaluation time.
 - `evalSystems`: The set of systems for which `nixpkgs` should be evaluated. Defaults to the four official platforms (`x86_64-linux`, `aarch64-linux`, `x86_64-darwin` and `aarch64-darwin`).

ci/eval/default.nix

···
           "nixos"
           "pkgs"
           ".version"
-          "ci/supportedSystems.nix"
+          "ci/supportedSystems.json"
         ]
       );
     };
 
   nix = nixVersions.nix_2_24;
 
-  supportedSystems = import ../supportedSystems.nix;
+  supportedSystems = builtins.fromJSON (builtins.readFile ../supportedSystems.json);
 
   attrpathsSuperset =
     runCommand "attrpaths-superset.json"
···
           nix
           time
         ];
-        env.supportedSystems = builtins.toJSON supportedSystems;
-        passAsFile = [ "supportedSystems" ];
       }
       ''
         export NIX_STATE_DIR=$(mktemp -d)
···
             --option restrict-eval true \
             --option allow-import-from-derivation false \
             --arg enableWarnings false > $out/paths.json
-        mv "$supportedSystemsPath" $out/systems.json
       '';
 
   singleSystem =

ci/supportedSystems.json

···
+[
+  "aarch64-linux",
+  "aarch64-darwin",
+  "x86_64-linux",
+  "x86_64-darwin"
+]

ci/supportedSystems.nix

···
-[
-  "aarch64-linux"
-  "aarch64-darwin"
-  "x86_64-linux"
-  "x86_64-darwin"
-]

pkgs/top-level/release-haskell.nix

···
   $ hydra-eval-jobs -I . pkgs/top-level/release-haskell.nix
 */
 {
-  supportedSystems ? import ../../ci/supportedSystems.nix,
+  supportedSystems ? builtins.fromJSON (builtins.readFile ../../ci/supportedSystems.json),
 }:
 
 let

pkgs/top-level/release-outpaths.nix

···
   attrNamesOnly ? false,
 
   # Set this to `null` to build for builtins.currentSystem only
-  systems ? import ../../ci/supportedSystems.nix,
+  systems ? builtins.fromJSON (builtins.readFile ../../ci/supportedSystems.json),
 }:
 let
   lib = import (path + "/lib");

pkgs/top-level/release.nix

···
   system ? builtins.currentSystem,
   officialRelease ? false,
   # The platform doubles for which we build Nixpkgs.
-  supportedSystems ? import ../../ci/supportedSystems.nix,
+  supportedSystems ? builtins.fromJSON (builtins.readFile ../../ci/supportedSystems.json),
   # The platform triples for which we build bootstrap tools.
   bootstrapConfigs ? [
     "aarch64-apple-darwin"