Migrate mediawiki-core-npm to Zuul cloner

The Jenkins git plugin is legacy, migrate to zuul cloner.  As a side
effect that fix up a bug when cleaning submodules fetched from a wmf
branch and a master branch build is run.

Adjust the npm-in path since core is now fetched under /src/

Bug: 71314
Change-Id: I81a8a6487ef150119e2a8aa470c4cba9fd68ca4a