gnu: guile-next: Really depend on self.

Previously, when cross-compiling, "self" would point to GUILE-3.0
because it came from (package-native-inputs guile-3.0).

When cross-compiling, ./configure would see a version in 'guile-3.0.pc'
different from its own version and would thus bail out:

  building Guile 3.0.8-1.cabbage but `/gnu/store/…/bin/guile' has version 3.0.7

* gnu/packages/guile.scm (guile-next)[native-inputs]: Replace "self".

gnu/packages/guile.scm

@@ -477,7 +477,8 @@ without requiring the source code to be rewritten.")
                   flex
                   gnu-gettext
                   texinfo
-                  gperf)))
+                  gperf)
+         (replace "self" this-package)))
       (synopsis "Development version of GNU Guile"))))
 
 (define* (make-guile-readline guile #:optional (name "guile-readline"))