Create GitHub Actions matrix from nix flake show

.github/workflows/build-flake-outputs.yml

@@ -0,0 +1,43 @@
+name: Build flake outputs
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  nix-matrix:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v24
+      - id: set-matrix
+        name: Generate matrix
+        run: |
+          matrix=$(
+            nix flake show --refresh --json "." |
+              jq '{ attr: ((path(.. | select(.type? == "derivation")) | map((if (. | contains(".")) then "\"" + . + "\"" else . end)) | join(".")),
+                           (path(.. | select(.type? == "nixos-configuration")) | map((if (. | contains(".")) then "\"" + . + "\"" else . end)) | join(".") | . + ".config.system.build.toplevel"))
+                  }' |
+              jq --slurp --compact-output
+          )
+          echo "matrix=$matrix" >> "$GITHUB_OUTPUT"
+
+  nix-build:
+    needs: nix-matrix
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include: ${{fromJSON(needs.nix-matrix.outputs.matrix)}}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v24
+      - uses: cachix/cachix-action@v12
+        with:
+          name: matrss
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+      - run: nix build -L '.#${{ matrix.attr }}'