nix-shell: Play nicely with non-interactive shells

nix-shell with the --command flag might be used non-interactively, but
if bash starts non-interactively (i.e. with stdin or stderr not a
terminal), it won't source the script given in --rcfile. However, in
that case it *will* source the script found in $BASH_ENV, so we can use
that instead.

Also, don't source ~/.bashrc in a non-interactive shell (detectable by
checking the PS1 env var)

Signed-off-by: Shea Levy <shea@shealevy.com>