Who's this random somebody that has a copy of your code that you don't want shared or trained on? GitHub's no saint here, but it seems like that somebody is an issue, not GitHub.
Until now there was never a reason to have a permissive software license that tries to restrict where the recipients can store the code, because this is a new issue. My software is AGPLv3 but I'd rather just stop sharing my code entirely than let these corpos profit from my work for free. Mirroring onto GitHub is very very common since GitHub is frequently accessible in network environments where random others forges may not be.