Update Cygwin mirror URL

[anmonteiro]

old one seems down

[a2line]

I confirm Windows CI for Geneweb are failing on setup-ocaml@v3 because https://mirrors.kernel.org/ is down since about 36h. Edit: main cygwin mirror is back!

[spotandjake]

This might make a good case to add a cygwin_source: string parameter to the action that by default takes the main mirror and allows us to override it with something like the sonic mirror.

[smorimoto]

I’ll get this merged right away, but honestly, the source was way more unstable than the mirror ever was.

[Alizter]

Why not have a fallback?

[smorimoto]

Thank you for raising this. I'll have a think about adding fallback logic, but given that kernel.org is built on one of the most robust infrastructures available and it's back up now, I'm inclined to keep using it as the primary mirror going forward.