cbuildbot: hardcode new main branch for luci config

The git branch resolution logic is finding the wrong value, so hardcode
"main" to unbreak the builder until we can track this down.

BUG=chromium:1126855, chromium:1179447
TEST=CQ passes

Change-Id: Ibf7cbda6353900c3e6c94dd50628d05c55485e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2703111
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Navil Perez <navil@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Navil Perez <navil@google.com>
diff --git a/cbuildbot/stages/config_stages.py b/cbuildbot/stages/config_stages.py
index fe4cb5c..3bfbd42 100644
--- a/cbuildbot/stages/config_stages.py
+++ b/cbuildbot/stages/config_stages.py
@@ -485,7 +485,7 @@
     git.RunGit(
         self.project_dir, ['config', 'push.default', 'tracking'],
         print_cmd=True)
-    git.PushBranch('HEAD', self.project_dir, dryrun=self._run.options.debug)
+    git.PushBranch('main', self.project_dir, dryrun=self._run.options.debug)
     logging.PrintBuildbotStepText('luci-scheduler.cfg: Updated.')
 
   def PerformStage(self):