build: Expand ‘scripts/guix’ at Make time.

This moves the complexity of Autotools variable expansion outside of the
application code.

* scripts/guix.in (config-lookup): Delete.
(maybe-augment-load-paths!, run-guix-main): Use fully expanded variables
instead of calling ‘config-lookup’.
* configure.ac: Don't use AC_CONFIG_FILES for ‘scripts/guix’.
Use AC_PROG_SED.
* Makefile.am (scripts/guix): New rule.
(do_subst): New variable.
(CLEANFILES, EXTRA_DIST): Adapt.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>