refactor: remove `REPO_UNDER_TEST`

Replace the `REPO_UNDER_TEST` variable with the `GERRIT_PROJECT`
variable from the Gerrit Trigger Jenkins plugin.

Change-Id: I6d34d91ce07b07dc77dc17efdd8d0ff6230426ee
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed