It's free, so why not?
This also partially deprecates the Docker container used for building -
now it's used only for MXE builds (because building MXE takes way too
long to do it inside the action)
This is pretty much a straight port of our Jenkinsfile to GitHub
Actions. It could use some modernization in a few places, but it should
work for now.
Note that I left the Jenkinsfile for now - we'll be using them both
until we are sure that the GitHub Actions version is stable.
MrSimbax