Installer improvements

[Andrew Arnott]

• Offer the ARM64 app. We already build it, and it works. But we can't mix it into the github release assets, so we need to push to a new location (e.g. Amazon S3).
• Offer delta update support, which may require downloading (all?) prior releases to the pipeline before using squirrel --releasify. We may want to do this in the release pipeline instead of the build one.
• Offer channels (e.g. CI, release).
• Improve the UI within the app to show download progress.
• Update Squirrel to offer a net5.0 target, or replace the in-proc facility with something like Orc.Squirrel.