os/exec: revert macOS __fork fix before point release

[rsc]

Per #57263 (comment) it has its own unidentified problems. At this point the safest path forward for the point releases is to just revert the __fork fix. We can deal with Go 1.20 separately (as #57263).

[rsc]

@gopherbot backport

[rsc]

@gopherbot please backport WTF

[gopherbot]

Backport issue(s) opened: #57689 (for 1.18), #57690 (for 1.19).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[rsc]

Closing this issue, which I created only to trigger backport issues. #57263 will serve as the Go 1.20 issue.