update github actions default from master to main
update github actions default from master to main

Summary: Branches updated some time ago, update this to match

Reviewed By: zpao

Differential Revision: D32693531

fbshipit-source-id: 02da79e3fb5fcd3b204be4605521a151b977ce2c

nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman