bug#48223: EXWM knows nothing about Guix profiles

[Leo Prikler]

Hi Guix,

I just recently helped someone debug an issue they encountered when
using EXWM.  It turned out their EMACSLOADPATH was set to
"/run/current-system/profile/share/emacs/site-
lisp:/gnu/store/1zszglsxl4zxy9alcwxjwj26d30qmyv9-emacs-
27.2/share/emacs/27.2/lisp" – quite notably, "$HOME/share/emacs/site-
lisp" was missing.

I think the launcher that we install in the install-xsession does not
do sufficient work to set up the environment variables of the session
appropriately.  In particular, I think it should source /etc/profile
prior to running Emacs.

WDYT?