Skip building when already in cache

.github/workflows/build.yml

@@ -20,6 +20,8 @@ jobs:
   prepare-arm64-runner:
     #if: false
     runs-on: ubuntu-24.04
+    outputs:
+      cache-status: ${{ steps.cache-status.outputs.cache-status }}
     steps:
       - uses: cachix/install-nix-action@v30
         with:
@@ -29,14 +31,18 @@ jobs:
           name: packer-kubernetes-hcloud
           authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
       - uses: actions/checkout@v4
-      - run: nix run nixpkgs#nix-eval-jobs -- --gc-roots-dir gcroot --check-cache-status --flake '.#nixosConfigurations.install-image-orangepi5plus.config.system.build.toplevel'
+      - run: nix-env -iA nixpkgs.nix-eval-jobs nixpkgs.jq
+      - run: nix-eval-jobs --gc-roots-dir gcroot --check-cache-status --flake '.#nixosConfigurations.install-image-orangepi5plus.config.system.build.toplevel' | tee result.json
+      - id: cache-status
+        run: echo "cache-status=$(jq -r .cacheStatus result.json)" >> "$GITHUB_ENV"
       - uses: simonswine/hetzner-github-runner@HEAD
+        if: "${{ steps.cache-status.outputs.cache-status == 'notBuilt' }}"
         with:
           github-api-key: ${{ secrets.GH_API_KEY_RUNNER }}
           hetzner-api-key: ${{ secrets.HETZNER_API_KEY }}
           hetzner-instance-type: cax21
   build-arm64:
-    #if: false
+    if: "${{ needs.prepare-arm64-runner.outputs.cache-status == 'notBuilt' }}"
     runs-on: self-hosted
     needs: prepare-arm64-runner
     steps: