.github/workflows: per GHC step, gc after

.github/workflows/nixpkgs-ci.yml

@@ -8,12 +8,35 @@ on:
     branches: [ master ]
 
 jobs:
-  nix-build:
+  get-ghc-versions:
+    name: Determine GHC versions to test
     runs-on: ubuntu-latest
+    outputs:
+      versions: ${{ steps.get-ghc-versions-step.outputs.versions }}
+
     steps:
     - uses: actions/[email protected]
     - uses: cachix/install-nix-action@v30
       with:
         nix_path: nixpkgs=channel:nixos-unstable
-    - name: ci.nix
-      run: nix-build ci.nix
+    - name: Get GHC Versions from ci.nix
+      id: get-ghc-versions-step
+      run: |
+        echo -n "versions=" >> "$GITHUB_OUTPUT"
+        nix-instantiate --eval --strict --json -E 'builtins.attrNames (import ./ci.nix { })' >> "$GITHUB_OUTPUT"
+
+  nix-build:
+    runs-on: ubuntu-latest
+    needs:
+      - get-ghc-versions
+    strategy:
+      matrix:
+        version: ${{ fromJSON(needs.get-ghc-versions.outputs.versions) }}
+    - uses: actions/[email protected]
+    - uses: cachix/install-nix-action@v30
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - name: nix-build ci.nix -A ${{ matrix.version }}
+      run: |
+        nix-build -A ${{ matrix.version }}
+        nix-store --gc