GitHub Actions CI: Don't skip intermediate builds on master; only cancel intermediate builds on PR builds (#909)

[skip tests]

Author: DilumAluthge

.github/workflows/UnitTests.yml

@@ -22,10 +22,10 @@ on:
       - 'lib/**'
 
 concurrency:
-  # Skip intermediate builds: always.
-  # Cancel intermediate builds: always.
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
+  # Skip intermediate builds: all builds except for builds on the `master` branch
+  # Cancel intermediate builds: only pull request builds
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref != 'refs/heads/master' || github.run_number }}
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 
 jobs:
   test-default: