release.nix: namespace bootstrap tools with triples

This will allow buliding bootstrap tools for platforms with
non-default libcs, like *-unknown-linux-musl.

This gets rid of limitedSupportSystems/systemsWithAnySupport. There
was no need to use systemsWithAnySupport for supportDarwin, because it
was always equivalent to supportedSystems for that purpose, and the
only other way it was used was for determining which platforms to
build the bootstrap tools for, so we might as well use a more explicit
parameter for that, and then we can change how it works without
affecting the rest of the Hydra jobs.

Not affecting the rest of the Hydra jobs is important, because if we
changed all jobs to use config triples, we'd end up renaming every
Hydra job. That might still be worth thinking about at some point,
but it's unnecessary at this point (and would be a lot of work).

I've checked by running

nix-eval-jobs --force-recurse pkgs/top-level/release.nix

that the actual bootstrap tools derivations are unaffected by this
change, and that the only other jobs that change are ones that depend
on the hash of all of Nixpkgs. Of the other jobset entrypoints that
end up importing pkgs/top-level/release.nix, none used the
limitedSupportedSystems parameter, so they should all be unaffected as
well.

maintainers/scripts/all-tarballs.nix

···
     scrubJobs = false;
     # No need to evaluate on i686.
     supportedSystems = [ "x86_64-linux" ];
-    limitedSupportedSystems = [];
+    bootstrapConfigs = [];
   }

pkgs/stdenv/darwin/make-bootstrap-tools.nix

···
   };
 
   bootstrapTools = derivation {
-    inherit (localSystem) system;
+    inherit (stdenv.hostPlatform) system;
 
     name = "bootstrap-tools";
     builder = "${bootstrapFiles.tools}/bin/bash";

pkgs/top-level/release.nix

···
 */
 { nixpkgs ? { outPath = (import ../../lib).cleanSource ../..; revCount = 1234; shortRev = "abcdef"; revision = "0000000000000000000000000000000000000000"; }
 , officialRelease ? false
-  # The platforms for which we build Nixpkgs.
+  # The platform doubles for which we build Nixpkgs.
 , supportedSystems ? [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ]
-, limitedSupportedSystems ? [ "i686-linux" ]
+  # The platform triples for which we build bootstrap tools.
+, bootstrapConfigs ? [
+    "aarch64-apple-darwin"
+    "aarch64-unknown-linux-gnu"
+    "i686-unknown-linux-gnu"
+    "x86_64-apple-darwin"
+    "x86_64-unknown-linux-gnu"
+  ]
   # Strip most of attributes when evaluating to spare memory usage
 , scrubJobs ? true
   # Attributes passed to nixpkgs. Don't build packages marked as unfree.
···
 
 let
 
-  systemsWithAnySupport = supportedSystems ++ limitedSupportedSystems;
-
   supportDarwin = lib.genAttrs [
     "x86_64"
     "aarch64"
-  ] (arch: builtins.elem "${arch}-darwin" systemsWithAnySupport);
+  ] (arch: builtins.elem "${arch}-darwin" supportedSystems);
 
   nonPackageJobs =
     { tarball = import ./make-tarball.nix { inherit pkgs nixpkgs officialRelease supportedSystems; };
···
         };
 
       stdenvBootstrapTools = with lib;
-        genAttrs systemsWithAnySupport (system:
-          if hasSuffix "-linux" system then
+        genAttrs bootstrapConfigs (config:
+          if hasInfix "-linux-" config then
             let
               bootstrap = import ../stdenv/linux/make-bootstrap-tools.nix {
                 pkgs = import ../.. {
-                  localSystem = { inherit system; };
+                  localSystem = { inherit config; };
                 };
               };
             in {
               inherit (bootstrap) dist test;
             }
-          else if hasSuffix "-darwin" system then
+          else if hasSuffix "-darwin" config then
             let
               bootstrap = import ../stdenv/darwin/make-bootstrap-tools.nix {
-                localSystem = { inherit system; };
+                localSystem = { inherit config; };
               };
             in {
               # Lightweight distribution and test
···
               #inherit (bootstrap.test-pkgs) stdenv;
             }
           else
-            abort "No bootstrap implementation for system: ${system}"
+            abort "No bootstrap implementation for system: ${config}"
         );
     };