distro: guile: Patch (ice-9 popen) to use the right shell.

* distro/packages/guile.scm (guile-2.0): Add `pre-configure'.
* distro/packages/base.scm (guile-final): Adjust to preserve the
  `pre-configure' phase.

distro/packages/base.scm

@@ -969,8 +969,10 @@ store.")
   ;; Libtool, so that that dependency is isolated in the "bin" output.
   (let ((guile (package (inherit guile-2.0/fixed)
                  (arguments
-                  `(#:phases
-                    (alist-cons-before
+                  (substitute-keyword-arguments
+                      (package-arguments guile-2.0/fixed)
+                    ((#:phases phases)
+                     `(alist-cons-before
                        'patch-source-shebangs 'delete-encoded-test
                        (lambda* (#:key inputs #:allow-other-keys)
                          ;; %BOOTSTRAP-GUILE doesn't know about encodings other
@@ -982,7 +984,7 @@ store.")
                            (lambda (p)
                              (format p "#!~a/bin/bash\nexit 77"
                                      (assoc-ref inputs "bash")))))
-                     %standard-phases))))))
+                       ,phases)))))))
     (package-with-bootstrap-guile
      (package-with-explicit-inputs guile
                                    %boot4-inputs

distro/packages/guile.scm

@@ -120,6 +120,16 @@ extensible.  It supports many SRFIs.")
 
    (self-native-input? #t)
 
+   (arguments
+    '(#:phases (alist-cons-before
+                'configure 'pre-configure
+                (lambda* (#:key inputs #:allow-other-keys)
+                  (let ((bash (assoc-ref inputs "bash")))
+                    (substitute* "module/ice-9/popen.scm"
+                      (("/bin/sh")
+                       (string-append bash "/bin/bash")))))
+                %standard-phases)))
+
    (synopsis "GNU Guile 2.0, an embeddable Scheme implementation")
    (description
 "GNU Guile is an implementation of the Scheme programming language, with