Rollup merge of #144196 - ChrisDenton:init-mingw, r=mati865

Initialize mingw for the runner's user

This is apparently the more proper fix to https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Spurious.20bors.20CI.20failures/near/528915775

But let's see if it works.

Author: matthiaskrgr