calendar-cli: add meta.mainProgram

nixos/tests/radicale.nix

···
   port = "5232";
   filesystem_folder = "/data/radicale";
 
-  cli = "${pkgs.calendar-cli}/bin/calendar-cli --caldav-user ${user} --caldav-pass ${password}";
+  cli = "${lib.getExe pkgs.calendar-cli} --caldav-user ${user} --caldav-pass ${password}";
 in {
   name = "radicale3";
   meta.maintainers = with lib.maintainers; [ dotlambda ];

pkgs/tools/networking/calendar-cli/default.nix

···
     description = "Simple command-line CalDav client";
     homepage = "https://github.com/tobixen/calendar-cli";
     license = licenses.gpl3Plus;
+    mainProgram = "calendar-cli";
     maintainers = with maintainers; [ dotlambda ];
   };
 }