khal: Use version from unstable

The version from stable throws errors and sometimes even crashes due to
an incompatibility with pytz.

users/simon/modules/sway/waybar.nix

@@ -232,7 +232,7 @@ in
 
 
         def khal(args):
-            completed = subprocess.run(["${pkgs.khal}/bin/khal"] + args, capture_output=True)
+            completed = subprocess.run(["khal"] + args, capture_output=True)
             assert completed.returncode == 0
             return completed.stdout.decode("utf-8")
 

users/simon/modules/vdirsyncer.nix

@@ -68,7 +68,7 @@ let
 in
 {
   home.packages = with pkgs; [
-    khal
+    unstable.khal
     khard
     vdirsyncer
   ];