Ensure older workflow runs for a pull-request are cancelled

The new `concurrency` setting can be used to ensure only a single
workflow run or job is in progress. When used in combination with the
`cancel-in-progress` setting, incomplete workflow runs can be cancelled
automatically to prevent running workflows unnecessarily.

Author: hashhar

.github/workflows/ci.yml

@@ -6,6 +6,13 @@ on:
       - "master"
   pull_request:
 
+# Cancel previous PR builds.
+concurrency:
+  # Cancel all workflow runs except latest within a concurrency group. This is achieved by defining a concurrency group for the PR.
+  # Non-PR builds have singleton concurrency groups.
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.event.number || github.sha }}
+  cancel-in-progress: true
+
 jobs:
   checks:
     runs-on: ubuntu-latest