**Apache Airflow version**:
2.0.1

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

**Environment**:

- **Cloud provider or hardware configuration**: 4GB RAM, Processor - Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
- **OS** (e.g. from /etc/os-release): 18.04.5 LTS (Bionic Beaver)
- **Kernel** (e.g. `uname -a`): Linux 4.15.0-136-generic
- **Install tools**: bare metal installation as per commands given [here](https://airflow.apache.org/docs/apache-airflow/stable/installation.html)

**What happened**:

On running `airflow scheduler -p` got following error:-
```
Traceback (most recent call last):
  File "/home/vineet/Documents/projects/venv/bin/airflow", line 8, in <module>
    sys.exit(main())
  File "/home/vineet/Documents/projects/venv/lib/python3.6/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/home/vineet/Documents/projects/venv/lib/python3.6/site-packages/airflow/cli/cli_parser.py", line 48, in command
    return func(*args, **kwargs)
  File "/home/vineet/Documents/projects/venv/lib/python3.6/site-packages/airflow/utils/cli.py", line 86, in wrapper
    metrics = _build_metrics(f.__name__, args[0])
  File "/home/vineet/Documents/projects/venv/lib/python3.6/site-packages/airflow/utils/cli.py", line 118, in _build_metrics
    full_command[idx + 1] = "*" * 8
IndexError: list assignment index out of range
```
As per docs, `-p` flag is a valid flag and gives correct result for `airflow scheduler --do-pickle`

**How to reproduce it**:
Install `airflow` 2.0.1 and run `airflow scheduler -p`
airflow/utils/cli.py
"""Expands path to absolute by replacing 'DAGS_FOLDER', '~', '.', etc."""
    if subdir:
        if not settings.DAGS_FOLDER:
            raise ValueError("DAGS_FOLDER variable in settings should be filled.")
        subdir = subdir.replace('DAGS_FOLDER', settings.DAGS_FOLDER)
        subdir = os.path.abspath(os.path.expanduser(subdir))
    return subdir
def get_dag_by_file_location(dag_id: str):
    """Returns DAG of a given dag_id by looking up file location"""
    from airflow.models import DagBag, DagModel
    
    dag_model = DagModel.get_current(dag_id)
    if dag_model is None:
        raise AirflowException(
            'dag_id could not be found: {}. Either the dag did not exist or it failed to '
            'parse.'.format(dag_id)
        )
    dagbag = DagBag(dag_folder=dag_model.fileloc)
    return dagbag.dags[dag_id]
def get_dag(subdir: Optional[str], dag_id: str) -> "DAG":