Track Cygwin repository as an input to the pipeline

[dra27]

I just manually triggered a rebuild of the Windows opam starting images in order to pick up Cygwin 3.3.0. Apparently, I hadn't had enough caffeine to remember that it would merrily download setup, see it was unchanged, and use the cache!

We should track the Cygwin repository as an input - the versions of key packages should be seen as automatically triggering rebuilds of the Windows images (gcc, binutils, Cygwin itself, etc.).

This could of course be generalised to other distros which operate rolling releases.