Update macOS build process

[Stereo]

Hi,

​https://github.com/openstreetmap/josm/pull/65/files describes the changes required to get actual macOS releases built on GitHub Actions. ​https://github.com/openstreetmap/josm/pull/65.diff is the machine-readable diff.

Because the tests are currently failing, the action has been configured to only run on macos, and to build automatically instead of waiting for the tests to fail.

​https://github.com/thomersch/josm/releases has a few test releases. If the build number isn't the same as tested, the releases get marked as pre-releases.

The next step will be to trigger some kind of update on the main wiki page when a release is actually built.