fix(pty_proc/macOS): Properly set the environment for the child

Binding _NSGetEnviron()'s return value to a local variable and then
re-binding that is incorrect.  We need to directly update what
_NSGetEnviron() refers to.

src/nvim/os/pty_process_unix.c

@@ -159,7 +159,7 @@ static void init_child(PtyProcess *ptyproc)
   FUNC_ATTR_NONNULL_ALL
 {
 #if defined(HAVE__NSGETENVIRON)
-  char **environ = *_NSGetEnviron();
+#define environ (*_NSGetEnviron())
 #else
   extern char **environ;
 #endif