Detect nix shell (in addition to nix-shell)

[dudeofawesome]

The nix_shell item currently detects whether it is inside a nix shell by looking at IN_NIX_SHELL, but that var is not set when using the new nix command's nix shell.

Some discussion in that issue talks about searching for nix store entries in PATH (which is summarized here).

I think Tide should adopt a similar strategy for detecting nix shell usage as well.

Prior art:

• https://github.com/starship/starship/blob/fccf4ccebf1533989234ec13c5a0f6d44e47e937/src/modules/nix_shell.rs#L32-L40