Fix environment variable parsing (apache#423)

* Fix environment variable parsing

```bash
export PYICEBERG_CATALOG__SOMETHING__S3__REGION=eu-north-1
```

Before:

```python
>>> from pyiceberg.catalog import load_catalog
>>> load_catalog('something').properties
{'s3': {'region': 'eu-north-1'}, ...}
```

After:

```python
>>> from pyiceberg.catalog import load_catalog
>>> load_catalog('something').properties
{'s3.region': 'eu-north-1', ...}
```

Which correspondents with the key `s3.region` that we use.

* Add second test

Co-authored-by: Hussein Awala <hussein@awala.fr>

* lint

---------

Co-authored-by: Hussein Awala <hussein@awala.fr>

Python: Fix caching of FileSystem (#8549)

Before:
```
python3 /tmp/benchmark.py

import: 0.175605 seconds
catalog: 0.060079 seconds
table: 0.245859 seconds
scan: 5.218688 seconds
Rows: 119890
```

After:
```
python3 /tmp/benchmark.py

import: 0.169811 seconds
catalog: 0.054512 seconds
table: 0.233786 seconds
scan: 2.165105 seconds
Rows: 119890
```