Separate McHelper into a separate GitHub action from the Main CI

Suggested by @kwcantrell in #519. Documenting in a new issue for reference.

Having McHelper in a new action would make it a lot easier for us / for users to distinguish when builds are failing due to the actual tests/etc. failing or due to McHelper being down. It should be possible, also, to set things up so that the "McHelper action" would only be triggered for a given push / PR / etc. when the main CI action passes -- so this shouldn't lose any of the current functionality available. Notes on this from #519 below:

[...] I thought it'd make sense to only upload stuff to McHelper when the Main CI passes (and we know that at least the tests are passing). This would avoid polluting McHelper with a bunch of broken QZVs.

...But thinking about it more, it doesn't have to be an all-or-nothing thing though -- it looks like it should be possible to configure GitHub actions to only run the separate McHelper action when the Main CI action (and maybe the standalone CI too?) passes. I think that might require a decent amount of messing around to get everything working right [...]