gnu: emacs-transient: Update to commit a6e4cced.

This is necessary to build a recent version of emacs-forge.

* gnu/packages/emacs-xyz.scm (emacs-transient): Update to commit a6e4cced.
[build-system]: Switch to the emacs-build-system.
[arguments]: Remove the #:modules, #:imported-modules and #:make-flags
arguments.
[phases]{configure}: Do not delete.
{install}: Do not replace.
{build-info-manual}: New phase.
{enter-lisp-directory}: New phase.
[native-inputs]: Remove emacs-minimal, which is implicitly provided by the
emacs-build-system.