Make "nq" (nix-env helper command) available in interactive shells

Currently the nq helper command (or, shell function) is placed in
/etc/profile (because that's where environment.shellInit contents go).
/etc/profile is only sourced by login shells and that means that nq has
not been available in (other) interactive shells.

Fix by moving the nq() function to /etc/bashrc (by using
environment.interactiveShellInit).