Bug 1788026 - Run browsertime tests nightly on the mozilla-beta branch. r=sparky, a=test-only

There will be changes in mozilla-beta that will increase the frequency of pushes. This will increase the load on our android devices so this patch will limit the amount of tests to be run nightly rather than on every push.

Differential Revision: https://phabricator.services.mozilla.com/D156198