Create a custom cygwin mirror

[embray]

The official cygwin mirrors feature "rolling releases" where every time new packages are updated only the previous one or two version remain available. It's also not possible with the Cygwin setup's CLI to specify package version to install.

This makes creating reproducible builds very difficult, since the Cygwin version itself, and the versions of all packages, are constantly shifting around.

I have in the past made some updates to the Makefile to allow installing Cygwin from a locally cached mirror that doesn't update as often. However, it would be nice to have a centrally located (e.g. in a git repo) partial Cygwin mirror (of only the packages needed for building/running Sage) that can be pointed to for building Sage for Windows in a more stable manner.

@mkoeppe This would probably help with the GH CI as well.

[slel]

There is a Cygwin "time-machine" archive at

• http://www.crouchingtigerhiddenfruitbat.org/Cygwin/timemachine.html