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
1 file changed
tree: 1271a4b3cdb2c7c1998cdfb1535735e29dc66da2
  1. .gitreview
  2. LICENSE
  3. README
  4. playbooks/
  5. roles/
  6. zuul.d/