Cancel correctly tasks from sync context in Scheduler

When stopping the executing from ee, which runs in another thread, we need to use the correct loop
when cancelling the job tasks. Further, we just signal to cancel therefore we don't need to await
for the tasks to finish. This is handled in the Scheduler.execute - asyncio.gather.

src/ert/scheduler/scheduler.py

@@ -62,6 +62,8 @@ def __init__(
         }
 
         self._events: asyncio.Queue[Any] = asyncio.Queue()
+        self._loop = asyncio.get_running_loop()
+
         self._average_job_runtime: float = 0
         self._completed_jobs_num: int = 0
         self.completed_jobs: asyncio.Queue[int] = asyncio.Queue()
@@ -77,6 +79,12 @@ def __init__(
 
     def kill_all_jobs(self) -> None:
         self._cancelled = True
+        assert self._loop.is_running()
+        asyncio.run_coroutine_threadsafe(
+            self._signal_kill_all_jobs(), self._loop
+        ).result()
+
+    async def _signal_kill_all_jobs(self) -> None:
         for task in self._tasks.values():
             task.cancel()
 
@@ -166,8 +174,7 @@ async def execute(
                 )
 
             start.set()
-            for task in self._tasks.values():
-                await task
+            await asyncio.gather(*self._tasks.values())
 
             await self.driver.finish()