Well, it seems the environment is changing so we'd better run the tests
reguarly to see whether something breaks in a timely manner.
GitHub Actions also don't offer other Windows environments now, only
windows-latest is available. Reduce test matrix to only that.
Try compiling some C code, If it works then we're probably fine.
I don't even try to verify cross-compilation because I don't need it.
It should work, but that's not my problem right now.
Again, this is a squashed commit with around three hours of attempts
to write something working. See comments in the file for more details.
I'm too lazy to document this stuff properly.
Now that I know how to write some simple action, let's make a stub
more complete. For example, I already know what inputs I would like
to handle for this action.
ilammy