qa: fix head ref

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

.github/workflows/qa.yml

@@ -11,7 +11,7 @@ jobs:
             -   uses: actions/checkout@v3
                 with:
                     repository: ${{ github.event.pull_request.head.repo.full_name }}
-                    ref: $GITHUB_HEAD_REF
+                    ref: ${{ github.event.pull_request.head.ref }}
             -   name: set up JDK 11
                 uses: actions/setup-java@v2
                 with: