**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): N/A

**Environment**: 

- **Cloud provider or hardware configuration**: local/aws
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04.5 LTS
- **Kernel** (e.g. `uname -a`): 5.4.0-1032-aws
- **Install tools**: pip
- **Others**:

**What happened**:
I did a fresh Airflow 2.0.0 install. With this version, when I manually trigger a DAG, Airflow skips the next scheduled run.
<!-- (please include exact error messages if you can) -->

**What you expected to happen**:
Manual runs do not interfere with the scheduled runs prior to Airflow 2. 
<!-- What do you think went wrong? -->

**How to reproduce it**:
Create a simple hourly DAG. After enabling it and the initial run, run it manually. It shall skip the next hour. Below is an example, where the manual run with execution time of 08:17 causes the scheduled run with execution time of 08:00 to skip. 
![image](https://user-images.githubusercontent.com/26160471/103457719-c7193480-4d12-11eb-82cb-42efaedc9ef4.png)


tests/jobs/test_scheduler_job.py
"""Test SLA Callbacks are not sent when no task SLAs are defined"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_no_task_slas'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag)
        with patch.object(settings, "CHECK_SLAS", True):
            scheduler_job = SchedulerJob(subdir=os.devnull)
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_not_called()
    def test_send_sla_callbacks_to_processor_sla_with_task_slas(self):
        """Test SLA Callbacks are sent to the DAG Processor when SLAs are defined on tasks"""
        dag_id = 'test_send_sla_callbacks_to_processor_sla_with_task_slas'
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE, schedule_interval='@daily')
        DummyOperator(task_id='task1', dag=dag, sla=timedelta(seconds=60))
        with patch.object(settings, "CHECK_SLAS", True):
            scheduler_job = SchedulerJob(subdir=os.devnull)
            mock_agent = mock.MagicMock()
            scheduler_job.processor_agent = mock_agent
            scheduler_job._send_sla_callbacks_to_processor(dag)
            scheduler_job.processor_agent.send_sla_callback_request_to_execute.assert_called_once_with(
                full_filepath=dag.fileloc, dag_id=dag_id
            )
    def test_scheduler_sets_job_id_on_dag_run(self):