installer: Fix crash in parameters menu.

Fixes: <https://issues.guix.gnu.org/45378>.

* gnu/installer/newt/keymap.scm (run-layout-page): Return false when
"Continue" button is pressed.
(run-keymap-page): Do not try to compute variants if layout is false. Also do
not try to format the result if layout is false.
* gnu/installer.scm (compute-keymap-step): Do not apply keymap if
"run-keymap-page" returns false.