build: emacs-build-system: Unify the installation directory.

build: emacs-build-system: Unify the installation directory.

This change aims to reduce the length of the EMACSLOADPATH environment
variable, which was found to cause issues such as
<https://bugs.gnu.org/38309>.

It should also enable discovery of newly installed packages without refreshing
the session's EMACSLOADPATH of the user profile (e.g., when launching Emacs
from the desktop manager application launcher), as discussed in
<https://bugs.gnu.org/38309>.

* guix/build/emacs-build-system.scm (%legacy-install-suffix): Rename to...
(%install-dir): ...this.
(%install-suffix): Remove variable.
(build): Adjust installation target directory.
(patch-el-files): Likewise.
(install): Likewise.
(move-doc): Likewise.
(make-autoloads): Likewise.

Signed-off-by: Clément Lassieur <clement@lassieur.org>