install: Add nvi to the image.

* gnu/system/install.scm (installation-os)[packages]: Add NVI.
* doc/guix.texi (Proceeding with the Installation): Mention it.

doc/guix.texi

@@ -6394,8 +6394,9 @@ builds to @file{/gnu/store} which, initially, is an in-memory file system.
 
 Next, you have to edit a file and
 provide the declaration of the operating system to be installed.  To
-that end, the installation system comes with two text editors: GNU nano
-(@pxref{Top,,, nano, GNU nano Manual}), and GNU Zile, an Emacs clone.
+that end, the installation system comes with three text editors: GNU nano
+(@pxref{Top,,, nano, GNU nano Manual}), GNU Zile (an Emacs clone), and
+nvi (a clone of the original BSD @command{vi} editor).
 We strongly recommend storing that file on the target root file system, say,
 as @file{/mnt/etc/config.scm}.  Failing to do that, you will have lost your
 configuration file once you have rebooted into the newly-installed system.

gnu/system/install.scm

@@ -35,6 +35,7 @@
   #:use-module (gnu packages grub)
   #:use-module (gnu packages texinfo)
   #:use-module (gnu packages compression)
+  #:use-module (gnu packages nvi)
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-26)
   #:export (self-contained-tarball
@@ -401,6 +402,7 @@ Use Alt-F2 for documentation.
                      ;; space; furthermore util-linux's fdisk is already
                      ;; available here, so we keep that.
                      bash-completion
+                     nvi                          ;:wq!
                      %base-packages))))
 
 ;; Return it here so 'guix system' can consume it directly.