distro: Add `guile-2.0/fixed', which should rarely change.

* distro/packages/guile.scm (guile-2.0/fixed): New variable.
* distro/packages/base.scm (guile-final): Use it.

distro/packages/base.scm

@@ -920,7 +920,7 @@ store.")
   ;; holds a dependency on the bootstrap Binutils.  Use multiple outputs for
   ;; Libtool, so that that dependency is isolated in the "bin" output.
   (package-with-bootstrap-guile
-   (package-with-explicit-inputs guile-2.0
+   (package-with-explicit-inputs guile-2.0/fixed
                                  %boot4-inputs
                                  (current-source-location)
                                  #:guile %bootstrap-guile)))

distro/packages/guile.scm

@@ -130,6 +130,11 @@ call interface, and powerful string processing.")
    (home-page "http://www.gnu.org/software/guile/")
    (license "LGPLv3+")))
 
+(define-public guile-2.0/fixed
+  ;; A package of Guile 2.0 that's rarely changed.  It is the one used
+  ;; in the `base' module, and thus changing it entails a full rebuild.
+  guile-2.0)
+
 
 ;;;
 ;;; Extensions.