Allow overriding via env var path to .gitcookies in gerrit_util.py

This is to make it easy to specify a different .gitcookies for bots.

BUG=skia:5979

Change-Id: Iaeb148d25f1c2c3d06ab5e1b155999566ef5f294
Reviewed-on: https://chromium-review.googlesource.com/413075
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Ravi Mistry <rmistry@google.com>

gerrit_util.py

@@ -156,6 +156,8 @@ class CookiesAuthenticator(Authenticator):
 
   @classmethod
   def get_gitcookies_path(cls):
+    if os.getenv('GIT_COOKIES_PATH'):
+      return os.getenv('GIT_COOKIES_PATH')
     return os.path.join(os.environ['HOME'], '.gitcookies')
 
   @classmethod