xsession: source /etc/profile at the beginning of the script

The xsession script runs services that depend on a sane environment. Gpg-agent, for
example, runs the program "pinentry-gtk-2" to obtain the password to unlock GnuPG
and SSH keys. That program will display only gibberish unless $FONTCONFIG_FILE is
configured properly. Instead of configuring these variables explicitly one by one,
we just source /etc/profile, which contains the appropriate @shellInit@ code.

modules/services/x11/display-managers/default.nix

@@ -21,6 +21,7 @@ let
     ''
       #! /bin/sh
 
+      . /etc/profile
       cd "$HOME"
 
       # The first argument of this script is the session type.