ci: OPS-2024: Fix concurrency on main (#4258)

Signed-off-by: Dillon Cullinan <dcullinan@nvidia.com>

.github/workflows/container-validation-backends.yml

@@ -11,7 +11,11 @@ on:
       - release/*.*.*
 
 concurrency:
-    group: ${{ github.workflow }}-build-test-${{ github.ref_name || github.run_id }}
+    # The group name is a ternary operation. If the ref_name is 'main',
+    # then the group name uses the run_id to ensure a unique group for
+    # 'main' pushes. Otherwise, the group name is the ref_name, so that
+    # workflows on the same PR/branch have the same group name for cancelling.
+    group: docker-build-test-${{ github.ref_name == 'main' && github.run_id || github.ref_name }}
     cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
 
 jobs: