ci(nix): switch to nix-quick-install-action for faster setup

- Replace cachix/install-nix-action with nixbuild/nix-quick-install-action
- Expected speedup: ~3 seconds per job (1s vs 4s for Nix installation)
- Remove github_access_token input (not needed for nix-quick-install-action)

Author: ryoppippi

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

@@ -1,5 +1,5 @@
 name: "Setup Nix"
-description: "Install Nix and configure cache"
+description: "Install Nix"
 inputs:
   tools:
     description: 'Space-separated list of nixpkgs packages to install (e.g., "uv ty just")'
@@ -13,9 +13,7 @@ runs:
   using: "composite"
   steps:
     - name: Install Nix
-      uses: cachix/install-nix-action@0b0e072294b088b73964f1d72dfdac0951439dbd # v31.8.4
-      with:
-        github_access_token: ${{ github.token }}
+      uses: nixbuild/nix-quick-install-action@2c9db80fb984ceb1bcaa77cdda3fdf8cfba92035 # v34
 
     - name: Install tools from nixpkgs
       shell: bash