Add support for the Cygwin Time Machine

[me-and]

Using the Cygwin Time Machine requires (a) being able to specify the
mirror that's used for installing Cygwin, and (b) being able to disable
the signature checks the installer performs. Add options that allow
that, tests to check it works as expected, and documentation for the new
options.

[jon-turney]

Thanks.

I assume you have a use for this?

My one concern is that setup can handle multiple --site options, specifying different package repositories. We don't need to handle that right now, but I'd like to understand how the site parameter could handle multiple URLs being specified.

[me-and]

Yep: my use case was wanting to do some Git builds with some old Cygwin packages, in the name of checking whether a regression was due to a change in the Git code or in one of the packaged Cygwin dependencies. Obviously it can be used to specify any mirror, not just a Time Machine one; I guess someone doing something unusual might want to use their own set of packages from their own mirror...

I'd forgotten it was possible to specify multiple mirrors, but I think that should be fine to handle. It looks like GitHub Actions doesn't allow a parameter to be anything other than a string, but I see no reason this couldn't be extended to accept multiple URLs with some separator (newlines seem the obvious choice, and is what I've seen other actions do, e.g. the path parameter for @actions/upload-artifact). There would just need to be some straightforward string wrangling to convert the input format to however setup.exe expects things, but this action is already doing similar wrangling with the packages parameter.

[jon-turney]

Thanks.

I created a v2 tag