nixos: fix sway screen glitches with vulkan

Author: contrun

nix/common.nix

@@ -802,6 +802,10 @@ in
       extraSessionCommands = ''
         # see also https://wiki.hyprland.org/Configuring/Environment-variables/
 
+        # Fix screen tearing in external display of machine with nvidia GPU
+        # https://old.reddit.com/r/swaywm/comments/102cdqa/how_can_i_fix_my_external_screen_flickering_with/
+        export WLR_RENDERER=vulkan
+
         export TERMINAL="alacritty"
         export BROWSER="firefox"