pr-checks: Try without setting checkout ref

Going to test this with new PR from Athanasius account, with a 'behind'
develop.

.github/workflows/pr-checks.yml

@@ -30,7 +30,7 @@ jobs:
       # https://github.com/actions/checkout
       uses: actions/checkout@v2
       with:
-        ref: ${{github.head.sha}}
+        #ref: ${{github.head.sha}}
         #repository: ${{github.event.pull_request.head.repo.full_name}}
         #fetch-depth: 0