[CI] Limit concurrency of build cache update jobs.

If several merges are happening in close succession, the same number of
build-cache update jobs will run (in parallel). It would be enough if
only one of these were to run.
Here, the update jobs for macOS are assigned to a concurrency group.
This ensures that only one job runs, and only one job is pending.
The pending job always gets replaced with the latest version when a
merge happens. For now, this is only in effect when the workflow is
triggered by a push, and only for macOS, but this could be extended.

Adapted from commit 1383734

(cherry picked from commit 270e43f)

Author: dpiparo

.github/workflows/root-ci.yml

@@ -88,6 +88,13 @@ jobs:
     permissions:
       contents: read
 
+    concurrency:
+      # We need only one build cache update to run after merges to master
+      # The group name expands to something like
+      # "ROOT CI-20964/merge-mac26-21173435944", unless it's a build cache update, where it will be
+      # "ROOT CI-master     -mac26-update_build_cache", so only one of these jobs will run
+      group: ${{ github.workflow }}-${{ github.ref_name }}-${{ matrix.platform }}-${{ github.event_name == 'push' && 'update_build_cache' || github.run_id }}
+
     strategy:
       fail-fast: false
       matrix: