perf(ci): switch to nix-quick-install-action for faster CI (#298)

Replace cachix/install-nix-action with nixbuild/nix-quick-install-action.
This reduces Nix installation time from ~30s to ~1s on Linux.

Key benefits:
- Single-user installation (no daemon overhead)
- Deterministic installation for a given action version
- Minimal setup optimized for CI environments

Measured improvement: Setup Nix step reduced from 25s to 19s (~24% faster)

Author: ryoppippi

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

@@ -13,9 +13,7 @@ runs:
   using: 'composite'
   steps:
     - name: Install Nix
-      uses: cachix/install-nix-action@4e002c8ec80594ecd40e759629461e26c8abed15 # v31.9.0
-      with:
-        github_access_token: ${{ github.token }}
+      uses: nixbuild/nix-quick-install-action@2c9db80fb984ceb1bcaa77cdda3fdf8cfba92035 # v34
 
     - name: Install tools from nixpkgs
       shell: bash