Improve GHA configuration when using NRN as boilerplate (add owner/repo constants)

[Vadorequest]

Currently, it's not obvious/easy to know all the constants that needs to be replaced in the GHA workflow files.

For example:

[click to see logs](https://github.com/UnlyEd/next-right-now/pull/${{ steps.pr_id_finder.outputs.number }}/checks)

This would become [click to see logs](https://github.com/XXX/YYY/pull/${{ steps.pr_id_finder.outputs.number }}/checks)

We could define constants on top of each workflow file that'd be used instead of those unnamed constants, and that'd make it easier to replace them all at once in a breeze.