credentials set through env vars are still not loaded

Follow-up from https://github.com/duckdblabs/duckdb_aws/issues/7

Tried with the latest code but I'm still seeing the behavior where env vars are not loaded by the extension.

Below steps are run when ~/.aws/credentials is verified to be empty but credentials are set using export as env vars:

stephaniewang@Stephanies-MacBook-Pro duckdb_aws % ./build/debug/duckdb
v0.8.2-dev2700 acbbfe0e79
Enter ".help" for usage hints.
Connected to a transient in-memory database.
Use ".open FILENAME" to reopen on a persistent database.
D select * from read_parquet('s3://motherduck-dev-test-bucket/pypi.small.parquet');
Error: HTTP Error: HTTP GET error on 'https://motherduck-dev-test-bucket.s3.amazonaws.com/pypi.small.parquet' (HTTP 403)
D install aws; load aws; load httpfs;
D CALL load_aws_credentials();
100% ▕████████████████████████████████████████████████████████████▏ 
┌──────────────────────┬──────────────────────────┬──────────────────────┬───────────────┐
│ loaded_access_key_id │ loaded_secret_access_key │ loaded_session_token │ loaded_region │
│       varchar        │         varchar          │       varchar        │    varchar    │
├──────────────────────┼──────────────────────────┼──────────────────────┼───────────────┤
│                      │                          │                      │ us-east-1     │
└──────────────────────┴──────────────────────────┴──────────────────────┴───────────────┘
D

duckdb / duckdb_aws

credentials set through env vars are still not loaded #9