gnu: installer: Launch the installer as kmscon login-program.

gnu: installer: Launch the installer as kmscon login-program.

Source /etc/environment just before starting the installer. The login program
is supposed to load the environment variables of this file through PAM, but as
we replace it by the installer, they are no longer available. This is mostly
useful for the LANG environment variable.

* gnu/installer/build-installer.scm (installer-program-launcher): New exported
procedure.
* gnu/system/install.scm (%installation-services): Restore most of the origin
code. kmscon is only started on TTY1, and the graphical installer is the
login-program.