.github: Parallelize cleanup of multicluster setup

When a GitHub job is cancelled, we have 5 minutes to perform any cleanup
action [1]. After that time, the job is forcefully stopped. That means
for jobs creating Kubernetes clusters, we have 5 minutes to delete the
cluster(s) or they will be left behind.

In the GKE test, it takes us around 2 minutes to delete the cluster. In
the multicluster test however, deleting the two GKE clusters takes us a
bit more than 5 minutes on average.

This commit parallelize the deletion of the two clusters in the
multicluster test to get the cleanup step below the 5-minutes runtime
threshold.

1 - https://docs.github.com/en/actions/managing-workflow-runs/canceling-a-workflow#steps-github-takes-to-cancel-a-workflow-run
Reported-by: Maciej Kwiek <maciej@isovalent.com>
Signed-off-by: Paul Chaignon <paul@cilium.io>

.github/workflows/multicluster.yaml

@@ -234,8 +234,16 @@ jobs:
       - name: Clean up GKE
         if: ${{ always() }}
         run: |
-          gcloud container clusters delete ${{ env.clusterName1 }} --zone ${{ env.zone }} --quiet
-          gcloud container clusters delete ${{ env.clusterName2 }} --zone ${{ env.zone }} --quiet
+          i=0
+          for e in ${{ env.clusterName1 }} ${{ env.clusterName2 }}; do
+            gcloud container clusters delete $e --zone ${{ env.zone }} --quiet &
+            pids[$i]=$!
+            ((i++))
+          done
+          # Wait for all cluster-cleanup processes.
+          for pid in ${pids[*]}; do
+            wait $pid
+          done
         shell: bash {0}
 
       - name: Upload artifacts