ci: add concurrency group to test-flags workflow (#606)

Prevents manual workflow_dispatch invocations from stacking on the
same ref. Uses cancel-in-progress: false to match ci.yml so a
reusable workflow_call invocation inside an in-flight CI run isn't
killed when a new push lands.

Co-authored-by: Sum Abiut <sum.abiut@titanfx.com>

.github/workflows/test-flags.yml

@@ -4,6 +4,12 @@ on:
   workflow_call: # Make this workflow reusable
   workflow_dispatch: # Allows manual triggering for testing
 
+concurrency:
+  group: test-flags-${{ github.ref }}
+  # Matches ci.yml: queue rather than cancel, so a reusable invocation
+  # from an in-flight CI run isn't killed mid-flight on the next push.
+  cancel-in-progress: false
+
 jobs:
   test-flags:
     runs-on: ubuntu-latest