Add initial GitHub Actions-based Windows CI
Add initial GitHub Actions-based Windows CI

Only a few tests are able to be run, because the integration test
support code fails to compile on Windows, taking effect before the
SKIP_INTEGRATION_TESTS env-var is handled.

Signed-off-by: Paul "TBBle" Hampson <220d126b3d70d9d7b0fda508e4cb236265bccc41@Pobox.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc