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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
gzip=False,
        google_impersonation_chain: Optional[Union[str, Sequence[str]]] = None,
        **kwargs,
    ):
        super().__init__(bucket=bucket, prefix=prefix, delimiter=delimiter, aws_conn_id=aws_conn_id, **kwargs)
        if dest_gcs_conn_id:
            warnings.warn(
                "The dest_gcs_conn_id parameter has been deprecated. You should pass "
                "the gcp_conn_id parameter.",
                DeprecationWarning,
                stacklevel=3,
            )
            gcp_conn_id = dest_gcs_conn_id
        self.gcp_conn_id = gcp_conn_id
        self.dest_gcs = dest_gcs
        self.delegate_to = delegate_to
        self.replace = replace
        self.verify = verify
        self.gzip = gzip
        self.google_impersonation_chain = google_impersonation_chain
        if dest_gcs and not gcs_object_is_directory(self.dest_gcs):
            self.log.info(
                'Destination Google Cloud Storage path is not a valid '
                '"directory", define a path that ends with a slash "/" or '
                'leave it empty for the root of the bucket.'
            )
            raise AirflowException(
                'The destination Google Cloud Storage path must end with a slash "/" or be empty.'
            )
    def execute(self, context):