test github actions with branch as var name

.github/workflows/python-app.yaml

@@ -62,7 +62,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install flake8 pytest
-        pip install -e git+https://github.com/wschuell/db_fillers.git@main#egg=db_fillers
+        pip install -e git+https://github.com/wschuell/db_fillers.git@$GITHUB_HEAD_REF#egg=db_fillers || pip install -e git+https://github.com/wschuell/db_fillers.git@main#egg=db_fillers 
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
         sudo apt-get install -y libsqlite3-dev
         pip install codecov pytest-cov