ui: Deduplicate 'show-what-to-build*'.

* guix/ui.scm (show-what-to-build*): New procedure.
* guix/scripts/environment.scm (show-what-to-build*): Delete.
* guix/scripts/system.scm (show-what-to-build*): Likewise.
* build-aux/make-binary-tarball.scm (show-what-to-build*): Likewise.