Windows CI is a mess due to cygwin.com instability

[Simn]

I'm seeing some version of this on every other run:

OpamDownload.Download_fail(_, "curl failed: \"D:\\\\cygwin\\\\bin\\\\curl.exe --write-out %{http_code}\\\\n --retry 10 --retry-delay 2 --user-agent opam/2.3.0 -L -o C:\\\\Users\\\\runneradmin\\\\AppData\\\\Local\\\\Temp\\\\opam-6100-4b6a36\\\\sha512.sum.part -- [https://cygwin.com/sha512.sum\](https://cygwin.com/sha512.sum/)" exited with code 16")

We need a solution for this...