popen: call /usr/bin/sh instead of /bin/sh

We mount /usr/bin to /bin, but in a chroot this is broken and we
have no /bin, so try to use the real path.

chroot is used by pacman to run install scripts when called with --root
and this broke programs in install scripts calling popen()
(install-info from texinfo for example)

There are more paths hardcoded to /bin in cygwin which might also be broken
in this scenario, so this maybe should be extended to all of them.

winsup/cygwin/syscalls.cc

@@ -4423,7 +4423,7 @@ popen (const char *command, const char *in_type)
       fcntl (stdchild, F_SETFD, stdchild_state | FD_CLOEXEC);
 
       /* Start a shell process to run the given command without forking. */
-      pid_t pid = ch_spawn.worker ("/bin/sh", argv, environ, _P_NOWAIT,
+      pid_t pid = ch_spawn.worker ("/usr/bin/sh", argv, environ, _P_NOWAIT,
 				   __std[0], __std[1]);
 
       /* Reinstate the close-on-exec state */