WIP: Move cx_Freeze build to GitHub Actions

[qstokkink]

Fixes #97

This PR is a WIP and the second attempt at switching (most of) the build process to GitHub Actions, now using cx_Freeze.

TODO (current focus):

• Figure out why cx_Freeze insists on adding garbage to the tar.gz.

[qstokkink]

I'll retire this attempt. TIL:

• Using the declarative format of cx_Freeze to exclude junk packages, like tests, is not easy. Using our current approach, of a dedicated makedist_debian.sh, seems like a fine way to go about this.
• Using the declarative format of cx_Freeze for cross-platform setup.py, will need a lot of platform-specific workarounds anyway.
• We can downsize our own code base using the autogeneration of cx_Freeze's bdist_deb, etc.
• Automatically retrieving the version from git using setuptools_scm is possible but probably more trouble (i.e., hidden caveats) than it's worth.

Most of the issues above can be fixed. However, the result would not be the elegant solution I'm looking for.