Prevent duplicate fnm_multishells in PATH

[teohhanhui]

Only eval fnm env in non-login shells.

The duplicate fnm_multishells in PATH leads to many problems, including a very broken fnm use system.

(This will of course also need to be fixed in the installer script as well...)

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[Schniz]

please explain this change further
it really de-simplifies the installation process and i don't get why you wouldn't want to have fnm env in a script, if you actually call it

[Schniz]

none of my scripts actually run fnm env but maybe because my scripts are in bash and i'm using zsh as shell

[teohhanhui]

The reason is that it's standard to source ~/.bashrc from ~/.bash_profile, e.g. the default in Fedora. (And it's also documented in bash documentation...)

If we don't have this check, it'll lead to duplicate fnm_multishells paths in PATH (once from login, once from the interactive shell). This means even if you try to disable fnm by fnm use system, it'd still fall back to the other instance in fnm_multishells, leading to errors like wrongly using the default version, or in some cases a non-functional node or npm.

[teohhanhui]

One possible alternative is to add a check to fnm env to output nothing if it detects fnm_multishells is already in PATH. (We can't just skip adding to PATH because that means we'll end up with a mismatch of which fnm_multishells instance is being used.)