ci: run on the pr branch

.github/workflows/ci.yml

@@ -10,7 +10,11 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
+    if: github.repository_owner == 'nix-community'
     steps:
     - uses: actions/checkout@v3
+      with:
+        # pull_request_target checks out the base branch by default
+        ref: refs/pull/${{ github.event.pull_request.number }}/merge
     - uses: cachix/install-nix-action@v18
     - run: nix build --extra-experimental-features nix-command -L -f . -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz --show-trace