use list comprehension instead of filter() to avoid pylint warning

```
************* Module salt.pillar.s3
salt/pillar/s3.py:247: [W0640(cell-var-from-loop), _refresh_buckets_cache_file.<lambda>] Cell variable saltenv defined in loop
```

More details: https://github.com/klen/pylama_pylint/blob/master/pylama_pylint/pylint/checkers/variables.py#L150