Update zshrc if it exists

[darichey]

It'd be nice if the script would also source /usr/sbin/start-systemd-namespace in .zshrc (or /etc/zsh/zshrc to match the existing use of bash.bashrc).

Right now, the behavior in zsh is very misleading:

~  λ snap list
error: access denied (try with sudo)
~  λ sudo snap list
error: access denied (see 'snap help login')

[EZaykov]

/etc/zsh/zshenv would be more accurate if u using ssh-key-chain for example