gnu: emacs-minimal: Install the custom site-start file.

This makes the EMACS-MINIMAL package more generally useful, by adding support
to load autoload definitions.  Note that this doesn't negatively impact the
performance of the emacs-build-system, because at that time the --quick option
is used, which disables loading site-start.el (which is the one calling
GUIX-EMACS-AUTOLOAD-PACKAGES).

* gnu/packages/emacs.scm (emacs-minimal)[phases]: Do not delete the
'install-site-start phase.
[inputs]: Add the emacs/guix-emacs.el auxiliary file.

gnu/packages/emacs.scm

@@ -336,13 +336,11 @@ (define-public emacs-minimal
     (build-system gnu-build-system)
     (arguments
      (substitute-keyword-arguments (package-arguments emacs)
-       ((#:phases phases)
-        `(modify-phases ,phases
-           (delete 'install-site-start)))
        ((#:configure-flags flags ''())
         `(list "--with-gnutls=no" "--disable-build-details"))))
     (inputs
-     `(("ncurses" ,ncurses)))
+     `(("guix-emacs.el" ,(search-auxiliary-file "emacs/guix-emacs.el"))
+       ("ncurses" ,ncurses)))
     (native-inputs
      `(("pkg-config" ,pkg-config)))))