installer: add option to not run zsh at the end

Co-authored-by: Liquidsoul <liquidsoul@liquidsoul.fr>
Co-authored-by: Alexander Polynomdivision <digitalmail555@googlemail.com>
Co-authored-by: loket <loket@cruftlab.io>
Co-authored-by: Connor Demille <subtlepseudonym@gmail.com>