build: 'script/guix' uses our own 'guile' executable.

* Makefile.am (do_subst): Substitute @abs_top_builddir@.
* scripts/guix.in: Use it.

Makefile.am

@@ -39,6 +39,7 @@ do_subst = $(SED)					\
   -e 's,[@]GUILE[@],$(GUILE),g'				\
   -e 's,[@]guilemoduledir[@],$(guilemoduledir),g'	\
   -e 's,[@]guileobjectdir[@],$(guileobjectdir),g'	\
+  -e 's,[@]abs_top_builddir[@],$(abs_top_builddir),g'	\
   -e 's,[@]localedir[@],$(localedir),g'
 
 scripts/guix: scripts/guix.in Makefile

scripts/guix.in

@@ -1,4 +1,4 @@
-#!@GUILE@ \
+#!@abs_top_builddir@/guile \
 --no-auto-compile -e main -s
 !#
 ;;; GNU Guix --- Functional package management for GNU