ci: try to enable cache for nix store

.forgejo/workflows/check.yml

@@ -7,6 +7,15 @@ jobs:
     steps:
       - name: Check out repository code
         uses: https://code.forgejo.org/actions/checkout@v4
+      - name: "Cache Nix store"
+        uses: actions/cache@v3.3.2
+        id: nix-cache
+        with:
+          path: /tmp/nixcache
+          key: "cache-nix-store"
+      - name: "Import Nix store cache"
+        if: "steps.nix-cache.outputs.cache-hit == 'true'"
+        run: "nix-store --import < /tmp/nixcache"
       - name: Prepare cachix
         uses: https://github.com/cachix/cachix-action@v12
         with:
@@ -15,3 +24,6 @@ jobs:
       - name: Run flake checks
         run: |
           nix --print-build-logs --verbose --accept-flake-config flake check
+      - name: "Export Nix store cache"
+        if: "steps.nix-cache.outputs.cache-hit != 'true'"
+        run: "nix-store --export $(find /nix/store -maxdepth 1 -name '*-*') > /tmp/nixcache"