Revert "switch to the macos-12-xl builder" This reverts commit fcbae989ae790d5b9a0a23ceba242d0b0d4e6c5b. This should fix the bors failures in https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Every.20rust-lang.2Frust.20PR.20is.20failing.20bors