TaskTree: Simplify withCancel() implementation

Don't use nested group, use conjunction instead.

Change-Id: Ifd4a6d89d17b3fc6d90f8dff60e2f997759ed5a5
Reviewed-by: hjk <hjk@qt.io>

src/libs/solutions/tasking/tasktree.cpp

@@ -1730,10 +1730,7 @@ Group ExecutableItem::withCancelImpl(
     return Group {
         parallel,
         stopOnSuccessOrError,
-        Group {
-            finishAllAndError,
-            BarrierTask(onSetup)
-        },
+        BarrierTask(onSetup) && errorItem,
         *this
     };
 }