Refactor workflow for proper MinGW support.

[kratz00]

With this PR Windows binaries based on MinGW are built and tested afterwards.

This PR should make the following PRs/issues obsolete and might be closed:

• https://github.com/twogood/unshield/pull/57
• https://github.com/twogood/unshield/pull/91
• https://github.com/twogood/unshield/issues/33