CI: Add Windows runner for Autotools builds with MSYS2.

CI: Add Windows runner for Autotools builds with MSYS2.

Only a subset of the tests run by the Linux and MacOS Autotools builds
are run. The most interesting tests are the ones that disable threads,
encoders, and decoders.

The Windows runner will only be run manually since these tests will
likely take much longer than the Linux and MacOS runners. This runner
should be used before merging any large features and before releases.

Currently the clang64 environment fails to due to a warning and
-Werror is enabled for the CI tests. This is still an early version
since the CMake build can be done for MSVC and optionally each of the
MSYS2 environments. GitHub does not allow manually running the CI tests
unless the workflow is checked on the default branch so checking in a
minimum version is a good idea.

Thanks to Arthur S for the original proposing the original patch.

Closes: https://github.com/tukaani-project/xz/pull/34