adding environment variables to /etc/config.scm

[Jonathan Lane]

Hello,

I have a laptop with Intel graphics and I need to set the environment
variable "COGL_ATLAS_DEFAULT_BLIT_MODE=framebuffer" to fix GPU 
corruption issues as described here:

https://wiki.archlinux.org/index.php/Intel_graphics#Troubleshooting
https://bugs.freedesktop.org/show_bug.cgi?id=88584

Unfortunately, it's supposed to go in /etc/environment, and that file
gets clobbered every time I reboot or run `guix system reconfigure`.  I
see that $GUIXSRC/gnu/system.scm does some logic to generate that file,
but I don't see an exported variable to manipulate from config.scm like
I do %desktop-services and the like.  What is the recommended way to add
global variables here?  It has to be a global variable because it alters
the behavior of the X server before gdm starts or my user logs in - I've
tested putting it in .profile and .bashrc to no avail.

Alternatively, this bug is not present on Wayland, so an update to make
gdm run via Wayland by default and expose Wayland sessions would solve
the root problem.

Thanks,

Jonathan Lane