[CI] Use regular runner on forks

[fabianschuiki]

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@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:

[teqdruid]

Unfortunately, not to my knowledge. I don't think there's any sort of 'fallback' runner, which is kinda stupid given that forks are a thing... Maybe it's a planned feature...?