I think the problem is that the build steps are executed inside a chroot created by the nix-daemon. So you'll have to do (add-after 'install 'install-javadocs ,install-javadocs) to make it available.
If this is used in more than just this file, you'll have to add the routine in a file inside guix/build/java-utils.scm and use #:modules and #:import-modules. See the qt-utils.scm file for a simple example and the greenisland or sddm packages that are on the mailinglist for how to use it. HTH!
