fish: Unset and set BROWSER

For some reason, BROWSER always gets set to firefox instead of
firefox-wayland which is our own desktop file for starting firefox
in wayland.

Setting the systemd environment variables or using the env in desktop
file has not helped.

fish/.config/fish/conf.d/sway.fish

@@ -3,3 +3,6 @@ set TTY1 (tty)
 if test -z "$DISPLAY"; and test $TTY1 = "/dev/tty1"
   exec sway
 end
+
+set -e BROWSER
+set -Ux BROWSER firefox-wayland