Use the lithium_rebase branch of RMG-database for CI

This is needed for testing.
Once lithium_rebase is merged to main on the database,
we can go back to using main.

.github/workflows/CI.yml

@@ -55,6 +55,11 @@ jobs:
     runs-on: macos-13
     # skip scheduled runs from forks
     if: ${{ !( github.repository != 'ReactionMechanismGenerator/RMG-Py' && github.event_name == 'schedule' ) }}
+    env:
+      # Update this if needed to match a pull request on the RMG-database:
+      RMG_DATABASE_BRANCH: lithium_rebase
+      # This is true only if this is a reference case for the regression testing:
+      REFERENCE_JOB: ${{ github.ref == 'refs/heads/main' && matrix.os =='ubuntu-latest' && github.repository == 'ReactionMechanismGenerator/RMG-Py' }}
     defaults:
       run:
         shell: bash -l {0}