1.2G had become slightly too small on x86_64.
This is a followup to 8dfb0c969e513276c632b8d26fb3601fa02993ca.
* gnu/tests/install.scm (%simple-installation-script)
(%extlinux-gpt-installation-script)
(%simple-installation-script-for-/dev/vda): Switch from 1.2G to 1.4G.