Setting `nix.package = pkgs.nix-monitored;` breaks nix-shell shebang

[leana8959]

As title, setting nix.package = pkgs.nix-monitored; breaks nix-shell shebangs.

Here's a minimal script you can use to test:

#!/usr/bin/env nix-shell
#!nix-shell -i bash

echo "within nix-shell"

The script would fail is the option is enabled, showing uninformative error.

$ ./test.sh
error: undefined variable 'echo'
       at /home/leana/.dotfiles/test.sh:4:1:
            3|
            4| echo "within nix-shell"
             | ^
            5|

[leana8959]

To fix this we can not set the nix.package option, but it would prevent the configuration provided in README from working. See #13 for a proposal to improve this.