Improve PARENT_BRANCH detection for pull request compatibility in CI workflow

Author: marevol

.github/workflows/maven.yml

@@ -18,7 +18,11 @@ jobs:
   build:
     runs-on: macos-14
     env:
-      PARENT_BRANCH: ${{ github.ref_name == 'master' && 'main' || github.ref_name }}
+      PARENT_BRANCH: ${{
+        github.event_name == 'pull_request' && github.event.pull_request.base.ref
+        || (github.ref_name == 'master' && 'main')
+        || github.ref_name
+      }}
     timeout-minutes: 15
     steps:
     - uses: actions/checkout@v4