repository: Override packaged repo cmd

The packaged repo command is not a full binary and as such does not
support some options, such as repo-rev, thus override the path to repo
for the R79 branch.  This was an oversight and I believe will allow us
to align R79 more with ToT.

BUG=b:179156096
TEST=`cros tryjob`

Change-Id: I931fd42fbfe6a3f36f6b27f761243eddabf5d82d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2673760
Commit-Queue: Mike Nichols <mikenichols@chromium.org>
Tested-by: Mike Nichols <mikenichols@chromium.org>
Auto-Submit: Mike Nichols <mikenichols@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
diff --git a/cbuildbot/repository.py b/cbuildbot/repository.py
index 8402460..a090c9b 100644
--- a/cbuildbot/repository.py
+++ b/cbuildbot/repository.py
@@ -394,6 +394,7 @@
       manifest_repo_url: A new value for manifest_repo_url.
       extra_args: Extra args to pass to 'repo init'
     """
+    self.repo_cmd = '/preload/chromeos/.repo/repo/repo'
     self.AssertNotNested()
 
     if manifest_repo_url: