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

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.