gerrit-sync-ve: need a ssh wrapper to have git use identity file

The mwext-VisualEditor-sync-gerrit Jenkins job is failling with:

 jenkins@gallium$ git push \
 ssh://jenkins-mwext-sync@gerrit.wikimedia.org:29418/mediawiki/extensions.git
HEAD:refs/for/master
 Permission denied (publickey).
 jenkins@gallium$

We have to pass the jenkins-mwext-sync identity file to git. That can be
done by specifying GIT_SSH but that env variable must point to a shell
script (it does not accept parameterized command).

Change-Id: I00701694f3307524e24f3c3d8d233725f0c24d88