ci: cache using nix-community/cache-nix-action

https://github.com/nix-community/cache-nix-action

.forgejo/workflows/check.yml

@@ -1,17 +1,54 @@
 name: Flake checks
 on: [pull_request]
+env:
+  USER: ci
 
 jobs:
   Check:
-    runs-on: nix-flakes
+    runs-on: ubuntu-latest
     steps:
       - name: Check out repository code
         uses: https://code.forgejo.org/actions/checkout@v4
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update && sudo apt-get install --yes zstd jq
+
+      - uses: https://github.com/nixbuild/nix-quick-install-action@v26
+        with:
+          load_nixConfig: false
+          nix_conf: |
+            substituters = https://cache.nixos.org/ https://nix-community.cachix.org
+            trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=
+            keep-outputs = true
+
+      - name: Calculate flake.lock hash
+        id: flake-lock-hash
+        run: |
+          echo "hash=$(md5sum flake.lock | awk '{print $1}')" >> $GITHUB_OUTPUT
+
+      - name: Restore and cache Nix store
+        uses: https://github.com/nix-community/cache-nix-action@v4
+        id: nix-store-cache
+        with:
+          key: cache-${{ runner.os }}-nix-store-${{ steps.flake-lock-hash.outputs.hash }}
+          restore-keys: |
+            cache-${{ runner.os }}-nix-store-
+
+          gc-linux: true
+          gc-max-store-size-linux: 10000000000
+
+          purge-caches: true
+          purge-keys: cache-${{ runner.os }}-nix-store-
+          purge-created: true
+          purge-created-max-age: 42
+
       - name: Prepare cachix
         uses: https://github.com/cachix/cachix-action@v12
         with:
           name: pub-solar
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
       - name: Run flake checks
         run: |
           nix --print-build-logs --verbose --accept-flake-config flake check