docs: update distributed-builds.md

[a-h]

Minor updates to remote building documentation.

Motivation

In Nix 2.24.9, nix store ping produces a warning that it is a deprecated alias, so I updated the commands.

warning: 'nix store ping' is a deprecated alias for 'nix store info'

I also added a list of requirements to assist with troubleshooting remote builds, and explained that it's the Nix daemon user that needs SSH access, not the interactive user.

Finally, I removed the $ at the start of the commands, and split the commands and output.

This is because the documentation site has a copy button for code samples, and if a $ is included, it is also copied to the clipboard along with the code, which is never what you want.