**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
import pytest
from parameterized import parameterized
from sqlalchemy import func
import airflow.example_dags
import airflow.smart_sensor_dags
from airflow import settings
from airflow.configuration import conf
from airflow.exceptions import AirflowException
from airflow.executors.base_executor import BaseExecutor
from airflow.jobs.backfill_job import BackfillJob
from airflow.jobs.scheduler_job import DagFileProcessor, SchedulerJob
from airflow.models import DAG, DagBag, DagModel, Pool, SlaMiss, TaskInstance, errors
from airflow.models.dagrun import DagRun
from airflow.models.serialized_dag import SerializedDagModel
from airflow.models.taskinstance import SimpleTaskInstance, TaskInstanceKey
from airflow.operators.bash import BashOperator
from airflow.operators.dummy import DummyOperator
from airflow.serialization.serialized_objects import SerializedDAG
from airflow.utils import timezone
from airflow.utils.callback_requests import DagCallbackRequest, TaskCallbackRequest
from airflow.utils.dag_processing import DagFileProcessorAgent
from airflow.utils.dates import days_ago
from airflow.utils.file import list_py_file_paths
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.utils.types import DagRunType
from tests.test_utils.asserts import assert_queries_count
from tests.test_utils.config import conf_vars, env_vars
from tests.test_utils.db import (
    clear_db_dags,