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**Apache Airflow version**:


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

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**: Docker

**What happened**:

When passing a templatized `dest_gcs` argument to the `S3ToGCSOperator` operator, the DAG fails to import because the constructor attempts to test the validity of the URL before the template has been populated in `execute`.

The error is:

```
Broken DAG: [/opt/airflow/dags/bad_gs_dag.py] Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/providers/google/cloud/hooks/gcs.py", line 1051, in gcs_object_is_directory
    _, blob = _parse_gcs_url(bucket)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/providers/google/cloud/hooks/gcs.py", line 1063, in _parse_gcs_url
    raise AirflowException('Please provide a bucket name')
airflow.exceptions.AirflowException: Please provide a bucket name
```

**What you expected to happen**:

The DAG should successfully parse when using a templatized `dest_gcs` value.

**How to reproduce it**:

Instantiating a `S3ToGCSOperator` task with `dest_gcs="{{ var.gcs_url }}"` fails. 

<details>

```python
from airflow.decorators import dag
from airflow.utils.dates import days_ago
from airflow.providers.google.cloud.transfers.s3_to_gcs import S3ToGCSOperator


@dag(
    schedule_interval=None,
    description="Demo S3-to-GS Bug",
    catchup=False,
    start_date=days_ago(1),
)
def demo_bug():

    S3ToGCSOperator(
        task_id="transfer_task",
        bucket="example_bucket",
        prefix="fake/prefix",
        dest_gcs="{{ var.gcs_url }}",
    )


demo_dag = demo_bug()
```
</details>


**Anything else we need to know**:

Should be fixable by moving the code that evaluates whether the URL is a folder to `execute()`.

airflow/providers/google/cloud/transfers/s3_to_gcs.py
"""
    Synchronizes an S3 key, possibly a prefix, with a Google Cloud Storage
    destination path.
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:S3ToGCSOperator`
    :param bucket: The S3 bucket where to find the objects. (templated)
    :type bucket: str
    :param prefix: Prefix string which filters objects whose name begin with
        such prefix. (templated)
    :type prefix: str
    :param delimiter: the delimiter marks key hierarchy. (templated)
    :type delimiter: str
    :param aws_conn_id: The source S3 connection
    :type aws_conn_id: str
    :param verify: Whether or not to verify SSL certificates for S3 connection.
        By default SSL certificates are verified.
        You can provide the following values:
        - ``False``: do not validate SSL certificates. SSL will still be used
                 (unless use_ssl is False), but SSL certificates will not be
                 verified.
        - ``path/to/cert/bundle.pem``: A filename of the CA cert bundle to uses.
                 You can specify this argument if you want to use a different
                 CA cert bundle than the one used by botocore.
    :type verify: bool or str
    :param gcp_conn_id: (Optional) The connection ID used to connect to Google Cloud.
    :type gcp_conn_id: str