add real nix cache

.gitea/workflows/deploy.yaml

@@ -10,7 +10,28 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - name: Restore and save Nix store
+        uses: nix-community/cache-nix-action@v6
+        with:
+          # restore and save a cache using this key
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          # if there's no cache hit, restore a cache by this prefix
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+          # collect garbage until the Nix store size (in bytes) is at most this number
+          # before trying to save a new cache
+          # 1G = 1073741824
+          gc-max-store-size-linux: 1G
+          # do purge caches
+          purge: true
+          # purge all versions of the cache
+          purge-prefixes: nix-${{ runner.os }}-
+          # created more than this number of seconds ago
+          purge-created: 60000
+          # or, last accessed more than this number of seconds ago
+          # relative to the start of the `Post Restore and save Nix store` phase
+          purge-last-accessed: 0
+          # except any version with the key that is the same as the `primary-key`
+          purge-primary-key: never
       - name: Install SSH key
         uses: shimataro/ssh-key-action@v2
         with: