bug#30350: 27.0.50; Newest master can't run processes on macOS

[Eli Zaretskii]

> From: Philipp <p.stephani2@gmail.com>
> Date: Sun, 04 Feb 2018 21:15:56 +0100
> 
> For some reason the newest master can't seem to start subprocesses on
> macOS:
> 
> emacs -batch -Q --eval='(call-process "/usr/bin/true")'
> Searching for program: Is a directory, /usr/bin/true
> 
> Needless to say, /usr/bin/true is a regular file.

Try reverting 327d251.