ssh/NixOS/nix-darwin: add `ssh.buildOnDestination`

[roberth]

Motivation

• Allow realisation to happen on the destination instead. Useful for bootstrapping an agent with the hci command.

• Modernize runNixOS a bit

• Docs improvements

Maintainer checklist

• [x] Documentation (function reference docs, setup guide, option reference docs)
• [x] Has tests (VM test, free account, and/or test instructions)
• [x] Is the corresponding module up to date?
  • n/a