Tweak installer test to just use dev branch

.github/workflows/test_installer.yaml

@@ -19,7 +19,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install package directly from the current repository
       run: |
-        pip install git+https://github.com/${{ github.repository }}@${{ github.event.ref }}
+        pip install git+https://github.com/${{ github.repository }}@$develop
     - name: Run post-installation test
       run: |
         python -c "from hed.models.hed_string import HedString; print('Import test passed.')"