Only set sccache_epilogue to run on build job exits (#67798)
Summary:
Fixes:
* https://github.com/pytorch/pytorch/issues/65431
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67798
Reviewed By: malfet
Differential Revision: D32174810
Pulled By: boyuantan
fbshipit-source-id: 072fdc042b56e541a877074120d41645c98e41f5
