**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)


airflow/models/dag.py
"""
        Set ``is_active=False`` on the DAGs for which the DAG files have been removed.
        Additionally change ``is_active=False`` to ``True`` if the DAG file exists.
        :param alive_dag_filelocs: file paths of alive DAGs
        :param session: ORM Session
        """
        log.debug("Deactivating DAGs (for which DAG files are deleted) from %s table ", cls.__tablename__)
        dag_models = session.query(cls).all()
        try:
            for dag_model in dag_models:
                if dag_model.fileloc is not None:
                    if correct_maybe_zipped(dag_model.fileloc) not in alive_dag_filelocs:
                        dag_model.is_active = False
                    else:
                        
                        
                        if not dag_model.is_active:
                            dag_model.is_active = True
                else:
                    continue
            session.commit()
        except Exception:
            session.rollback()
            raise
    @classmethod
    def dags_needing_dagruns(cls, session: Session):