cachix use suggests insecure trusted-users

[brainrake]

cachix use suggests to add your user to trusted-users in your system-wide nix config. However, that is unsafe as it can be used to run any command as root: NixOS/nix#9649 (comment)

Cachix should only recommend the first option.

Since the detection logic is already there, it could print a warning if a non-root user is in trusted-users.

Screenshot showing recommendation b) to add your user to trusted-users:

[brainrake]

Another option is to print a command that can be run to add the cache. It is easier to inspect.

[GeorgeFlerovsky]

Another option is to print a command that can be run to add the cache. It is easier to inspect.

This is much better than running something opaque as root.

[brainrake]

It could also show lines to add to NixOS configuration, works with flakes too, re. #533.

[brainrake]

Not sure how to avoid entering netrc contents on command line though.

[brainrake]

Related: #558