See if we can get stupid Slack to work

It really is brittle and dumb; it says it will log me in with a browser
and then go back to the app, but it cannot launch the app so it cannot
do anything at all.

configuration.nix

@@ -83,6 +83,10 @@
   ];
   environment.pathsToLink = [ "/share/fish" ];
 
+  # Needed for Slack but probably helps other stuff too.
+  # See https://wiki.nixos.org/wiki/Slack
+  environment.sessionVariables.NIXOS_OZONE_WL = "1";
+
   # Make it possible for ddev to modify the /etc/hosts file.
   # Otherwise you'll have to manually change the
   # hosts configuration after creating a new ddev project.