nixos/gnome/at-spi2-core: fix disabling a11y in all contexts

`environment.variables` gets sourced by shells & the x11 wrapper
through bash's `/etc/profile`, but not by systemd services, dbus
services, wayland sessions...

`environment.sessionVariables` sets these variables with PAM early in
the login process so it gets applied in all contexts.

I ran into a similar issue before in #109060.

nixos/modules/services/desktops/gnome/at-spi2-core.nix

···
     })
 
     (mkIf (!config.services.gnome.at-spi2-core.enable) {
-      environment.variables = {
+      environment.sessionVariables = {
         NO_AT_BRIDGE = "1";
         GTK_A11Y = "none";
       };