feat(emacs): Pin EXWM to 0.18

There are several severe regressions from EXWM 0.15 to EXWM 0.17. This
commit pins the version to the somewhat newer 0.18 in hope that it
resolves those issues again.

packages.nix

@@ -28,6 +28,8 @@ in {
       kontemplate = unstable.kontemplate;
       mq-cli = unstable.mq-cli;
 
+      # Override EXWM from 0.17 -> 0.18
+      pinnedEmacs.exwm = unstable.emacsPackagesNg.elpaPackages.exwm;
     };
   };