ci: include lockfile hash in cache key for proper invalidation

Add flake.lock and uv.lock hash to the cache key so that:
- Cache is saved when dependencies change (new hash = new key)
- Old cache is still restored via restore-prefixes-first-match
- Incremental updates build on previous cache

This ensures cache hits do not prevent saving updated derivations
while still benefiting from partial cache restoration.

Author: ryoppippi

.github/actions/setup-nix/action.yaml

@@ -16,7 +16,8 @@ runs:
     - name: Cache Nix store
       uses: nix-community/cache-nix-action@b426b118b6dc86d6952988d396aa7c6b09776d08 # v7
       with:
-        primary-key: nix-${{ runner.os }}-${{ inputs.python-version }}
+        primary-key: nix-${{ runner.os }}-${{ inputs.python-version }}-${{ hashFiles('flake.lock', 'uv.lock') }}
         restore-prefixes-first-match: |
+          nix-${{ runner.os }}-${{ inputs.python-version }}-
           nix-${{ runner.os }}-
         gc-max-store-size: 4G