Only run upgrade rust workflow on main repo

[LegNeato]

Fixes #344

[Firestar99]

@jcmnn Could I ask you to sync your rust-gpu fork? I'm getting emails every few days due to scheduled CI runs failing, you may get them as well. This PR makes them not run on forks, so syncing your main to rust-gpu's main should be enough. Thanks :D

(I could also ignore any mails from your repo in global settings, but if I ever push something to your repo again on some PR, I won't get mails for failing pipelines either.)

[jcmnn]

@jcmnn Could I ask you to sync your rust-gpu fork? I'm getting emails every few days due to scheduled CI runs failing, you may get them as well. This PR makes them not run on forks, so syncing your main to rust-gpu's main should be enough. Thanks :D

(I could also ignore any mails from your repo in global settings, but if I ever push something to your repo again on some PR, I won't get mails for failing pipelines either.)

Synced!