Make Unix.create_process_env thread-safe in all cases

[xavierleroy]

If neither spawnp nor execvpe are provided by the C library, the emulation code in otherlibs/unix, which is based on fork, ended up allocating memory in the child process using malloc.

This is not thread-safe: if the fork occurs while another OCaml domain or C thread is running inside malloc, the child process can inherit a memory state where it's unsafe to do malloc. For example, the mutex protecting malloc is locked, because it was locked at fork time, but the other thread is no longer there to finish the malloc and unlock the mutex, so malloc deadlocks in the child process.

This PR proposes a simple fix: in the child process, instead of calling our malloc-based emulation of execvpe, we just set environ to the desired environment and call execvp. This is safe because no other thread is running in the child process, so nobody can observe the change to the global variable environ.

Fixes: #12395.

[xavierleroy]

I did a round of precheck after disabling the posix_spawn path, so that our emulation is called on all platforms, and the results look good.

[nojb]

Neat trick! LGTM.