Sync workflow should work for forks

[bates64]

We have a "sync upstream" GitHub Actions workflow that generates PRs which merge an upstream repo into this one. For star-haven/papermario-dx (this repo), the upstream repo is pmret/papermario.

This is controlled by the UPSTREAM variable in the sync workflow

Currently, forks (i.e. mods) must manually update UPSTREAM to be star-haven/papermario-dx if they want to receive updates from us.

It would be great to do this automatically, e.g. by comparing GITHUB_ACTION_REPOSITORY == "star-haven/papermario-dx" to decide which upstream to use (pmret/papermario for star-haven/papermario-dx, or star-haven/papermario-dx otherwise). This would replace the UPSTREAM env variable entirely.