add manual workflow builds for linux and windows

[you-win]

~~Testing for the workflow is happening in this separate repo. A separate repo is being used since I find it easier to test new workflows like that.~~ Moved to my fork for final testing.

The idea is that precompiled binaries for Linux and Windows can be built using GitHub's hosted runners. For now, artifacts will just be uploaded as workload artifacts, but this can be configured to also create new releases.

Todos

• [ ] cache godot binary between runs and also maybe opencv on Windows targets
• [ ] fix building on windows, it looks like the opencv path that's being used is wrong
• [ ] verify uploading build artifacts works correctly

[j20001970]

Please note that #38 has been merged.