nixos/gdk-pixbuf: move GDK_PIXBUF_MODULE_FILE to sessionVariables

Some environments do not load /etc/profile, so environment.variables do
not work. In particular, this is the case for Plasma Wayland.

Use environment.sessionVariables for setting that variable instead,
which is handled by PAM and hence more reliable.

nixos/modules/services/x11/gdk-pixbuf.nix

@@ -21,7 +21,7 @@ in
   # loaders.cache based on that and set the environment variable
   # GDK_PIXBUF_MODULE_FILE to point to it.
   config = lib.mkIf (cfg.modulePackages != []) {
-    environment.variables = {
+    environment.sessionVariables = {
       GDK_PIXBUF_MODULE_FILE = "${loadersCache}";
     };
   };