Add MinGW CI

[IngwiePhoenix]

This is mainly for this and the mentioned issue: pcmacdon/jsish#36 (comment)

When V programs want to link to libraries that only compile under MinGW, it might be useful to test against mingw-w64 on Windows. A project might take advantage of autotools or the like to build a static library that a v program then links against.

Although I believe the issue mentioned in the issue is more related to Sokol, it's just a mere guess. Would be nice if testing was ran against that as well.