Only queue up one triage task at a time on CI (#3834)

Triage is scheduled to run once every 5 minutes but it's often queued up
during the day as builders are otherwise occupied with actual CI builds.
This can end up in a lot of triage tasks queued up back-to-back. While
this doesn't seem to be a huge issue one thing I suspect is that this is
perhaps somewhat related to API rate limits getting hit when recent
versions were published. In any case there's no need for each and every
triage run to do something, it's fine to only have one at a time
pending.

.github/workflows/triage.yml

@@ -10,6 +10,10 @@ on:
     # https://github.com/actions/labeler/issues/12
     - cron: '*/5 * * * *'
 
+concurrency:
+  group: issue-triage
+  cancel-in-progress: true
+
 jobs:
   triage:
     runs-on: ubuntu-latest