Have the ghc-head job checkout pr branches or ci/ghc-dev

Author: facundominguez

.github/workflows/ghc-head.yml

@@ -33,7 +33,8 @@ jobs:
     steps:
     - uses: actions/checkout@v4
       with:
-        ref: ci/ghc-head
+        # Checkout ci/ghc-head on scheduled runs, PR branch otherwise
+        ref: ${{ github.event_name == 'schedule' && 'ci/ghc-head' || github.event.pull_request.head.ref }}
         submodules: true
     - name: Install ghcup
       run: |