Prevent OSX updates to break fleek

[e-minguez]

Is your feature request related to a problem? Please describe.
When OSX is updated it restores back the /etc/zshrc file, making fleek not working.

Describe the solution you'd like
The usual workaround is to restore back the following lines at the /etc/zshrc level:

# Nix
if [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ]; then
  source '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'
fi
# End Nix

Describe alternatives you've considered
I guess the same can be done at ~/.zshrc level

Additional context
NixOS/nix#3616

Also NixOS/nix#3616 (comment)

[tulilirockz]

Having fleek autodetect if the system is a darwin one, then adding a function like "fix_osx_update()" in ~/.zshrc that writes the nix-importing line on /etc/zshrc should fix this issue

[bketelsen]

I think it might be better to use this fix:
NixOS/nix#3616 (comment)

I'll work on this soon, since the new update is rolling out.