build: only build tmux-terminfo when asked for

CI failed because Zsh couldn't be found on the Ubuntu image. We could
brew install it [1] like

    eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
    brew install zsh

but that has to be done conditionally, and it's not clear that putting
this target into the default install target is a good idea anyway.

[1]: https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2404-Readme.md

main.zuo

@@ -53,7 +53,7 @@
                   (for-each (lambda (f)
                               (alert (~a "  " f)))
                             '(git-extras brew-install rust)))]
-      [:target install (dirs symlink ,tmux-terminfo zcompile ,@features) ,void]
+      [:target install (dirs symlink zcompile ,@features) ,void]
       [:target update (pull-master update-submodules symlink brew-check vimtags rust zcompile) ,void]
       [:target vimtags ()
                ,(lambda (token)