git_cache: add OVERRIDE_BOOTSTRAP_BUCKET option.

Will use used by led-triggered tests of git_cache_updater builder.
Companion recipe CL: https://chromium-review.googlesource.com/c/infra/infra/+/1568156

R=hinoka

Change-Id: Ib25824d4d04cfb8aa0831fc5ae666a94808b289e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1567999Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_cache.py

@@ -245,9 +245,14 @@ class Mirror(object):
 
   @property
   def bootstrap_bucket(self):
+    b = os.getenv('OVERRIDE_BOOTSTRAP_BUCKET')
+    if b:
+      return b
     u = urlparse.urlparse(self.url)
     if u.netloc == 'chromium.googlesource.com':
       return 'chromium-git-cache'
+    # TODO(tandrii): delete once LUCI migration is completed.
+    # Only public hosts will be supported going forward.
     elif u.netloc == 'chrome-internal.googlesource.com':
       return 'chrome-git-cache'
     # Not recognized.