CI: try to add caching for nix build.

.github/workflows/ci.yml

@@ -150,6 +150,31 @@ jobs:
     - uses: cachix/install-nix-action@v31
       with:
         nix_path: nixpkgs=channel:nixos-unstable
+        nix_conf: |
+          keep-env-derivations = true
+          keep-outputs = true
+    - 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: 0
+        # or last accessed this duration (ISO 8601 duration format)
+        # before the start of the `Post Restore and save Nix store` phase
+        purge-last-accessed: P1DT12H
+        # except any version with the key that is the same as the `primary-key`
+        purge-primary-key: never
     - name: Cache cabal work
       id:   cabal-local
       uses: actions/cache@v5