doc: We now need 1 GiB for the installation USB image.

* doc/guix.texi (Building the Installation Image): Change image size to 1G.
* build-aux/hydra/gnu-system.scm (qemu-jobs): Likewise.

build-aux/hydra/gnu-system.scm

@@ -161,7 +161,7 @@ system.")
                        (set-guile-for-build (default-guile))
                        (system-disk-image installation-os
                                           #:disk-image-size
-                                          (* 860 MiB))))))
+                                          (* 1024 MiB))))))
       '()))
 
 (define (tarball-jobs store system)

doc/guix.texi

@@ -6056,8 +6056,9 @@ good.
 The installation image described above was built using the @command{guix
 system} command, specifically:
 
+@c FIXME: 1G is too much; see <http://bugs.gnu.org/23077>.
 @example
-guix system disk-image --image-size=850MiB gnu/system/install.scm
+guix system disk-image --image-size=1G gnu/system/install.scm
 @end example
 
 Have a look at @file{gnu/system/install.scm} in the source tree,