Multi-User fail if already has /nix

[DynamicApproach]

Motivation

If a user has already initiated and installed Nix as a single user, we should make sure they remove config files before attempting to install the multi-user variant.

Context

Rather than letting people double up on files, we should warn them to remove old files first.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

Add 👍 to pull requests you find important.

[abathur]

• This instruction won't work at all on macOS.
• Halting on the presence of /nix would also directly conflict with the effort to partially restore idempotence in Make multi-user installer idempotent #7603 (though I'm not sure if that effort will succeed).