.github: Parallelize cleanup of multicluster setup
.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 <33c1fdf481c8e628d4c6db7ea8dc77f49f2fa5d7@isovalent.com>
Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium