ENH: Allow trigger a nightly upload

I thought just triggering the run on main will upload the nightlies, but I don't think that is actually the case, due to `if: github.event_name == 'schedule' && github.repository == 'numpy/numpy-release'`

Occasionally it's nice to trigger the nightly upload to unblock downstream CI quicker.  It would be nice to add a checkbox for it or just always upload on main branch builds.