Cygwin: posix_spawnp: don't fallback to sh

Cygwin: posix_spawnp: don't fallback to sh

Per the discussion starting with
https://cygwin.com/pipermail/cygwin/2023-April/253495.html
stop falling back to sh if the file given to posix_spawnp
is no executable.

This is not necessarily the last word on it, given
https://www.austingroupbugs.net/view.php?id=1674, but for now,
opt for following the proposal in the Austin Group bug entry,
as well as PASSing the GNULIB test-posix_spawnp-script test.

Fixes: c7c1a1ca1b33 ("Add support for new posix_spawn function.")
Fixes: 3fbfcd11fb09 ("Cygwin: posix_spawn: add Cygwin-specific code fixing process synchronisation")
Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>