Make installing GitHub Actions build artifacts easier

[nickbianco]

It is becoming increasingly useful to give users the option to install "bleeding edge" build artifacts from opensim-core, especially for internal use. There are a few things that need to be improved:

• Artifacts are currently zipped twice. Users have to unzip twice, which is annoying, but also leads to super long filepaths which can cause issues on Windows.
• Mac artifacts are currently quarantined and must be de-quarantined via xattr -r -d com.apple.quarantine path/to/artifact to work. We should fine a way to automate this on GitHub Actions, or at least make it easier or users.
• We should find a way to automate uploading release builds to the GitHub release page.