Don't apply should_run to the nightly/postnightly branches. (#27061)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27061
Previously the cronjobs were run on master, but now the nightly builds
count as "PRs" so we must whitelist them from should_run calculation.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D17669066
Pulled By: ezyang
fbshipit-source-id: 3b92bf1d09aefa7ef524ea93dfa8c6f566161887
