ci: Use nixpkgs-unstable

Signed-off-by: Naomi Roberts <[e-mail]>

.tangled/workflows/deploy.yaml

···
 engine: "nixery"
 
 dependencies:
-  nixpkgs:
+  github:NixOS/nixpkgs/nixos-unstable:
+    - gleam
+    - erlang_28
+    - beam28Packages.rebar3
     - nodejs
 
 steps:
   - name: build site
     command: |
-      nix develop --command bash -c 'gleam run -m build'
+      gleam run -m build
   - name: deploy
     command: |
       npx --yes wrangler pages deploy ./priv --project-name naomieow-site --branch main