gnu: guix: Don't install in /etc/bash_completion.d within build environment.

Fixes <http://bugs.gnu.org/20394>.
Patch by Ludovic Courtès <ludo@gnu.org>.

* gnu/packages/package-management.scm (guix)[arguments]: Pass
  "--with-bash-completion-dir" option to configure.

gnu/packages/package-management.scm

@@ -67,6 +67,9 @@
      `(#:configure-flags (list
                           "--localstatedir=/var"
                           "--sysconfdir=/etc"
+                          (string-append "--with-bash-completion-dir="
+                                         (assoc-ref %outputs "out")
+                                         "/etc/bash_completion.d")
                           (string-append "--with-libgcrypt-prefix="
                                          (assoc-ref %build-inputs
                                                     "libgcrypt")))