fix(ci): add python-version to cache key for parallel jobs

Separate cache keys per Python version to prevent cache overwrites
when matrix jobs run in parallel. Default to python311 since it
matches the default devShell.

Author: ryoppippi

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

@@ -1,5 +1,10 @@
 name: "Setup Nix"
 description: "Install Nix and configure cache"
+inputs:
+  python-version:
+    description: "Python version for cache key (e.g., python311, python313)"
+    required: false
+    default: "python311"
 runs:
   using: "composite"
   steps:
@@ -11,5 +16,7 @@ runs:
     - name: Cache Nix store
       uses: nix-community/cache-nix-action@b426b118b6dc86d6952988d396aa7c6b09776d08 # v7
       with:
-        primary-key: nix-${{ runner.os }}
+        primary-key: nix-${{ runner.os }}-${{ inputs.python-version }}
+        restore-prefixes-first-match: |
+          nix-${{ runner.os }}-
         gc-max-store-size: 4G

.github/workflows/ci.yaml

@@ -46,6 +46,8 @@ jobs:
 
       - name: Setup Nix
         uses: ./.github/actions/setup-nix
+        with:
+          python-version: ${{ matrix.python-version }}
 
       - name: Run Lint
         run: nix develop .#${{ matrix.python-version }} --command just lint