gnu: installer: Fix issue with "Esperanto" locale.

According to glibc manual, locale are under the following form:
          language[_territory[.codeset]][@modifier]

The esperanto locale "epo" does not have a territory. Modify run-command to
take this into account.

Reported by Alex Sassmannshausen here:
https://lists.gnu.org/archive/html/guix-devel/2020-04/msg00192.html.

* gnu/installer/utils.scm (run-command): Handle locale without territory such
as "epo".

gnu/installer/utils.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2018 Mathieu Othacehe <m.othacehe@gmail.com>
+;;; Copyright © 2018, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -93,7 +93,8 @@ COMMAND exited successfully, #f otherwise."
           (setenv "LC_ALL" locale)
           (setenv "LANGUAGE"
                   (string-take locale
-                               (string-index locale #\_))))))
+                               (or (string-index locale #\_)
+                                   (string-length locale)))))))
 
   (guard (c ((invoke-error? c)
              (newline)