Added ref to checkout for pull request on GitHub dispatch (#3010)

# Description

The checkout reference for the GitHub dispatch action needed to be
update to specify PRs.

# Type of change
- [x] Bug fix (fixes something broken)
- [ ] New feature (adds functionality)
- [ ] Maintenance (code refactor, clean-up, new CI test, etc.)

Co-authored-by: Terry McGuinness <terry.mcguinness@noaa.gov>

.github/workflows/pw_aws_centos.yaml

@@ -29,6 +29,7 @@ jobs:
       with:
         path: ${{ github.run_id }}/HOMEgfs
         submodules: 'recursive'
+        ref: ${{ github.event.pull_request.head.ref }}
 
   build-link:      
     runs-on: [self-hosted, aws, parallelworks, centos]