Really define gerrit_root in copy-plugin-deps role

The copy-plugin-deps role relied on an undefined variable "gerrit_root".
This caused plugins such as oauth, which have external deps, to fail the
gerrit-plugin-build job.

It was already attempted in I68ef0074b69f, but prepare-gerrit-repos role
was adapted.

Bug: Issue 13037
Change-Id: Ife5dfbcdad28777916f69081cf4d3c094b021615