Automatically set fig settings pty.path without user intervention + make sure it's stable

[brendanfalk]

A lot of user's path variables are set incorrectly. They have to manually run fig settings pty.path "$PATH"

This is stupid and a waste of everyone's time.

We should make sure the $PATH variable in the pseudo terminal is always correct. Ideally, we don't want to be updating it too frequently as this is fragile and stuff could break.

Some ideas:

1. Prompt the user to run something once after their shell has loaded
2. Make 100% sure that Fig is the very very last thing sourced (probably hard to ensure and enforce)
3. Do it on fig source, fig update, or fig [something else] - maybe, but too frequent and this is fragile...
4. Work out a way of spawning a shell process in the user's selected shell, then getting all the env variables like we did last time... But investigate this and make it work properly.