Add concurrency group to sync-to-gerrit workflow

Add a top-level concurrency group (sync-to-gerrit) with
cancel-in-progress: false so that 5-minute scheduled cron runs coalesce
into at most one pending run when runners are busy, without cancelling
an in-progress sync.

Change-Id: I0a3e2970d6b983c1e728b6104df970b33a158900

.github/workflows/sync-to-gerrit.yaml

diff --git a/.github/workflows/sync-to-gerrit.yaml b/.github/workflows/sync-to-gerrit.yaml
index b267be9..50cbdd2 100644
--- a/.github/workflows/sync-to-gerrit.yaml
+++ b/.github/workflows/sync-to-gerrit.yaml
@@ -5,6 +5,10 @@
     - cron: '*/5 * * * *'
   workflow_dispatch:
 
+concurrency:
+  group: sync-to-gerrit
+  cancel-in-progress: false
+
 jobs:
   sync:
     runs-on: ubuntu-latest