This allows us to use texmf.cnf instead of having to set all required
environment variables manually.
* guix/build/texlive-build-system.scm (configure): New procedure.
(build): Simplify.
(%standard-phases): Add configure phase.
* guix/build-system/texlive.scm (texlive-build): Include (guix build union) in
modules.
(%texlive-build-system-modules): Likewise.