OpenBLAS

Run nightly Homebrew cron job only on the main repo, not on forks

I noticed this because GitHub emailed me that it would disable the
nightly job because it hadn't changed for 3 months. It currently takes
30-50 minutes daily, and by default runs on all forks of the main
repository that have the relevant workflow yaml file. That serves little
purpose and wastes quite a bit of energy - so disable the runs outside
of the main repo.

This will not disable the runs on forks already made in the past that
contain this workflow file, but it does save 3 months worth of runs on
every new fork that is created.

[skip ci]