wluma: set env variable to allow it to work

see:
https://github.com/maximbaz/wluma/issues/8

nixos/configuration.nix

@@ -125,6 +125,13 @@
   #  wget
   ];
 
+  # set global environment variables
+  environment.sessionVariables = {
+    # I don't know what this does, but it is needed for wluma to work correctly.
+    # It will also slightly increase cpu usage (on wayland compositors), but shouldn't be too bad.
+    WLR_DRM_NO_MODIFIERS = 1;
+  };
+
   # Some programs need SUID wrappers, can be configured further or are
   # started in user sessions.
   # programs.mtr.enable = true;