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?