Separate job for mediawiki-core-jslint to use git-mwcore scm

This way it can use the git-mwcore scm instead of the default one
which saves like 30 seconds (presumably this is because of the
workspace wiping and re-cloning, not because of submodules, as
those are disabled in both cases already).

Compare:

- https://integration.wikimedia.org/ci/job/mediawiki-core-jslint/11086/console

04:21:03 Checkout:mediawiki-core-jslint - hudson.remoting.Channel@48341675:gallium
04:21:03 Last Built Revision: Revision f06ffe5624e105e12e71e46fe5862b5ba6d53e0a (detached)
04:21:03 Wiping out workspace first.
04:21:03 Cloning the remote Git repository
04:21:03 Cloning repository git://zuul.eqiad.wmnet/mediawiki/core
04:21:03 git version 1.7.9.5
04:21:31 Fetching upstream changes from origin
04:21:32 Commencing build of Revision e7230e212b2a0042af0edf5dc442a5a7b90309c0 (detached)

- https://integration.wikimedia.org/ci/job/mediawiki-core-lint/34661/console

Change-Id: Ib5d46e584ccafe6d65b8831eb65f7e988991e7f7
04:19:09 Checkout:mediawiki-core-lint - hudson.remoting.Channel@48341675:gallium
04:19:09 Last Built Revision: Revision 281a846017e8d9bde2d1a018603aff581ee66a58 (detached)
04:19:09 Fetching changes from 1 remote Git repository
04:19:09 Fetching upstream changes from origin
04:19:11 Commencing build of Revision f06ffe5624e105e12e71e46fe5862b5ba6d53e0a (detached)