perf(startup): skip load of fifc rules when in interactive shell

[phanen]

I notice taht "fifc rules" is only intended to be used in a interative fish shell spawn by fzf (by $SHELL -c). So I tweak the startup process so that we can conditional skip it.

This pr can also fix https://github.com/gazorby/fifc/issues/31 in a more direct way.