fixup! ci-nix github action

.github/workflows/ci-nix.yaml

@@ -19,6 +19,27 @@ jobs:
     - name: Install nix
       uses: cachix/install-nix-action@v27
 
+    # https://github.com/marketplace/actions/cache-nix-store
+    - name: Restore and cache Nix store
+      uses: nix-community/cache-nix-action@v5
+      with:
+        # restore and save a cache using this key
+        # TODO: Also hash the cabal.config and *cabal files, since we depend on them
+        primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix') }}
+        # if there's no cache hit, restore a cache by this prefix
+        restore-prefixes-first-match: nix-${{ runner.os }}-
+        # collect garbage until Nix store size (in bytes) is at most this number
+        # before trying to save a new cache
+        gc-max-store-size-linux: 1073741824
+        # do purge caches
+        purge: true
+        # purge all versions of the cache
+        purge-prefixes: cache-${{ runner.os }}-
+        # created more than this number of seconds ago relative to the start of the `Post Restore` phase
+        purge-created: 0
+        # except the version with the `primary-key`, if it exists
+        purge-primary-key: never
+
     # # Possibly need to cache cabal.
     # - name: Cache .cabal
     #   uses: actions/cache@v3