ci(release): simplify nix setup by using nix develop directly

Replace setup-nix composite action with direct install-nix-action and
use nix develop for commands. This simplifies the release workflow by
leveraging the full devShell environment instead of manually installing
individual tools.

- Use install-nix-action directly instead of setup-nix composite action
- Remove skip-mock-server option (not needed with nix develop)

Author: ryoppippi

.github/workflows/release.yaml

@@ -24,11 +24,11 @@ jobs:
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         if: ${{ steps.release.outputs.release_created }}
 
-      - name: Setup Nix
+      - name: Install Nix
         if: ${{ steps.release.outputs.release_created }}
-        uses: ./.github/actions/setup-nix
+        uses: cachix/install-nix-action@0b0e072294b088b73964f1d72dfdac0951439dbd # v31.8.4
         with:
-          skip-mock-server: "true"
+          github_access_token: ${{ github.token }}
 
       - name: Update version in __init__.py
         if: ${{ steps.release.outputs.release_created }}