feat: add ephemeral Nix install action for GitHub runners

Author: jfroche

.github/workflows/nix-build.yml

@@ -23,14 +23,12 @@ jobs:
       - name: Checkout Repo
         uses: actions/checkout@v4
       - name: Install nix
-        uses: cachix/install-nix-action@v31
-        with:
-          install_url: https://releases.nixos.org/nix/nix-2.31.2/install
+        uses: ./.github/actions/nix-install-ephemeral
       - id: set-matrix
         name: Generate Nix Matrix
         run: |
           set -Eeu
-          echo matrix="$(nix shell nixpkgs/405fc615369e0ea1b9c284c107ca4c3e1bc15774#nix-eval-jobs --command scripts/github-matrix.py checks legacyPackages)" >> "$GITHUB_OUTPUT"
+          echo matrix="$(nix shell github:nix-community/nix-eval-jobs --command scripts/github-matrix.py checks legacyPackages)" >> "$GITHUB_OUTPUT"
 
   nix-build-aarch64-linux:
     name: ${{ matrix.name }} (aarch64-linux)
@@ -97,7 +95,7 @@ jobs:
   run-tests:
     needs: [nix-build-aarch64-linux, nix-build-aarch64-darwin] #, nix-build-x86_64-linux]
     if: |
-      !cancelled() &&
+      !cancelled() && 
       (needs.nix-build-aarch64-linux.result == 'skipped' || needs.nix-build-aarch64-linux.result == 'success') &&
       (needs.nix-build-aarch64-darwin.result == 'skipped' || needs.nix-build-aarch64-darwin.result == 'success')
     uses: ./.github/workflows/test.yml