Future Nix improvements

[RebeccaMahany]

After getting through everything that's required for initial NixOS support and rolling it out (#896), there are some items I'd like to come back to in order to improve maintainability and releases. This issue is for tracking those improvements.

• Some amount of automation for the release process -- since the rest of our package builds are automated, this would be nice to have: Automate the nix-agent releases #1726
• Test improvements -- use launchertest and/or CI in the nix-agent repo to validate nightly automatically
• Rewrite mock device/control server in Golang and move it to a more reusable location?
• Nix support on Darwin
• ARM support
• Make it easier to use allowUnfree -- addressed by Add overlay output, allow overriding NixOS module package, respect Nixpkgs config nix-agent#26