patch 8.2.3891: github CI: workflows may overlap

Problem:    Github CI: workflows may overlap.
Solution:   Cancel previous workflows when starting a new one. (Yegappan
            Lakshmanan, closes #9400)

.github/workflows/ci.yml

@@ -5,6 +5,13 @@ on:
     branches: ['**']
   pull_request:
 
+# Cancels all previous workflow runs for pull requests that have not completed.
+concurrency:
+  # The concurrency group contains the workflow name and the branch name for
+  # pull requests or the commit hash for any other events.
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+  cancel-in-progress: true
+
 jobs:
   linux:
     runs-on: ubuntu-18.04