bug#58734: installer: backtrace page in final step

[Mathieu Othacehe <othacehe@gnu.org>]

Hello,

When an error occurs in the pseudo terminal displaying the "guix system
init" command output, the backtrace is not displayed correctly and the
keyboard cannot be used (arrows, tab, enter keys) to scroll down the
backtrace or dump it.

It can easily be reproduced by introducing an error in the "run-command"
function, this way:

--8<---------------cut here---------------start------------->8---
--- a/gnu/installer/utils.scm
+++ b/gnu/installer/utils.scm
@@ -184,6 +184,7 @@ (define (pause)
       (((port _ ...) _ _)
        (read-line port))))
 
+  (error 'fake)
   (installer-log-line "running command ~s" command)
   (define result (run-external-command-with-line-hooks
                   (list %display-line-hook) command
--8<---------------cut here---------------end--------------->8---

I suspect that we may need to run "newt-init" and "clear-screen" before
displaying the backtrace page.

Thanks,

Mathieu