Stop hacking integration test from cloning across the network

We already have a local copy of the git repos in /opt/git and the
hacking integration test can use an arbitrary repo path by setting
REPO_ROOT. Since we don't need the target-project repo to super up to
date just use the local copy to stop cloning across the network.

Change-Id: I3c751934f082007e1bc86dc9ed4cc22afd79f369

jenkins/jobs/hacking.yaml

@@ -7,6 +7,7 @@
       - gerrit-git-prep
       - shell: |
           #!/bin/bash -xe
+          export REPO_ROOT=file:///opt/git
           tox -e integration {target-org} {target-project}
 
     publishers: