[CI] Use regular runner on forks

@teqdruid, I was wondering whether there's a way to only use the sponsored CI runner for `llvm/circt` builds, but fall back to the regular GitHub actions runner for forks. Do you know if that's a thing we can do? I realized that CI on my fork is busted because it also tries to run on the hosted runner :sweat_smile:

Start by inspecting the repository's GitHub Actions configuration to find where the sponsored runner is selected and how fork builds are handled. Verify that llvm/circt builds retain the sponsored runner while fork builds use a regular GitHub-hosted runner, then confirm the behavior with CI runs from both contexts.

Written by the indexing model from the issue text.