repo: only warn if update_depot_tools does not exist

It looks like some bots don't have this for whatever reason.

Bug: 1148984
Change-Id: I7ff826e0a0df78e3be9e1f84698b69574c75f71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2567384
Reviewed-by: Sean McAllister <smcallis@google.com>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>

repo

@@ -35,7 +35,11 @@ def _UpdateDepotTools():
       return
     path = path.parent
 
-  subprocess.run([UPDATE_DEPOT_TOOLS], check=True)
+  if UPDATE_DEPOT_TOOLS.exists():
+    subprocess.run([UPDATE_DEPOT_TOOLS], check=True)
+  else:
+    print(f'warning: {UPDATE_DEPOT_TOOLS} does not exist; export '
+          'DEPOT_TOOLS_UPDATE=0 to disable.', file=sys.stderr)
 
 
 def main(argv):