[backport-1.4] Do not rely on gitlab CI cache in delete workload cluster job

This is a backport of !8015 (merged)/!8058 (merged)

close #4103 (closed)

As common.sh can't be sourced outside of a .sh script we tweak a bit by running it from a temporary script. This is a quick workaround and we'll probably need to revisit common.sh at some point