"Invalid file open mode" error when using the GCS manifest in weather-dl

Can't run `weather-dl` with the GCS manifest type:

**Reproducing**

```bash
weather-dl configs/era5_example_config_local_run.cfg \
-m "gs://path/to/manifest.txt" \
--runner DataflowRunner \
--project \
--temp_location "gs://path/to/temp_dir" \
--job_name \
--region us-central1 \
```

**Error**
```
ValueError: Invalid file open mode: a.
```

The error is thrown [here](https://github.com/google/weather-tools/blob/de4fa216e428666d183a31155862db6f4420e424/weather_dl/download_pipeline/manifest.py#L202) which seems to be [expected](https://beam.apache.org/releases/pydoc/2.10.0/_modules/apache_beam/io/gcp/gcsio.html#:~:text=Invalid%20file%20open%20mode).