nixos: configure bash prompt in Emacs' shell-mode, even if TERM=dumb

The idea that the interactive bash prompt isn't set in case of TERM=dumb
is intended to fix problems when other machines log remotely into a
NixOS installation via Tramp. A side-effect that change was, however,
that Emacs' shell-mode no longer had a correct prompt. I suppose the
presence of

  INSIDE_EMACS=24.5.2,comint

is a sufficiently unique indication that the current interactive shell
is running inside of an Emacs and that the prompt can thus be configured
safely.