mediawiki-core-regression* jobs now use Zuul cloner

As a side effect, hhvm jobs are no more running concurrently just like
the PHP ones.   That is to avoid creating multiple workspaces on the
same instance.  Will be fixed later by throttling them.

Bug: 71549
Change-Id: I25c017ae99b9af34a270271f0851e3cca4407e36