sg: don't magically source .bashrc for keegan

Created by: keegancsmith

I think we do this because we mutate .bashrc in the sg setup and we try to validate if stuff is working after mutation. However, I don't use sg setup and I rely direnv to setup a special environment just for Sourcegraph.

This regression came from https://github.com/sourcegraph/sourcegraph/pull/31312

This commit exists as a talking point for the proper solution. Although I don't mind landing it either :)

Test Plan: sg works on my linux machine.