bug#62420: 29.0.60; fns-tests-collate-strings fails on Cygwin

[Eli Zaretskii]

> Date: Sat, 25 Mar 2023 11:14:57 -0400
> Cc: 62420@debbugs.gnu.org
> From: Ken Brown <kbrown@cornell.edu>
> 
> > What's the value of operating-system-release return in a Cygwin build?
> 
> Perfect!  It's "3.4.6-1.x86_64".  I should be able to use that. 
> Interestingly, that variable was made obsolete a couple years ago in 
> commit d88e12aa19c4.

We can un-obsolete it if Cygwin asks nicely ;-)

There's also the method used in report-emacs-bug--os-description,
maybe we could refactor it into a public function.