Disable sourcing .profile when checking for available updates

Implement once and for all #2431 by @feltnerm

> #### fix(tools/check_for_upgrade): Don't source profile
> Reverts #2296, but mostly #1883.
> 
> There is no need to source ~/.profile when this script is read. oh-my-zsh writes no configuration data in ~/.profile.
> 
> If the user wishes to use data within ~/.profile, then they should source it in another place.
> 
> Fixes #2315