Cross-platform release builds in CI

[robknight]

We should be able to produce cross-platform builds for MacOS, Linux, and Windows in GitHub CI.

[robknight]

This seems to work, subject to having proper code signing for MacOS.

[robknight]

Currently there are issues with the AppImage Linux build, although the RPM seems to work. I haven't tested the .deb but I assume that this works.

The AppImage problem seems to be a known issue with a potential fix: janhq/jan#5463

Testing the fix is somewhat difficult for me since I don't have an x86-64 Linux machine to test on right now.