AWS Access key is not being fetched from env variables

### Describe the bug

### Steps to reproduce
```yml
vars:
clickhouse_s3:
bucket: xx/xx
path: /**.parquet
fmt: Parquet
aws_access_key_id: "{{ env_var('AWS_ACCESS_KEY_ID') }}"
aws_secret_access_key: "{{ env_var('AWS_SECRET_ACCESS_KEY')}}"
```

or `select * from {{clickhouse_s3source('clickhouse_s3',aws_access_key_id='{{ env_var("AWS_ACCESS_KEY_ID") }}',aws_secret_access_key='{{ env_var("AWS_SECRET_ACCESS_KEY")}}')}}`

It wont pick up env var. If provided explicitly it works.

### Expected behaviour

Should work with variables

### Code examples, such as models or profile settings

### dbt and/or ClickHouse server logs

### Configuration
#### Environment
* dbt version:
* dbt-clickhouse version:
* clickhouse-driver version (if using native)
* clickhouse-connect version (if using http):
* Python version:
* Operating system:

#### ClickHouse server
* ClickHouse Server version:
* ClickHouse Server non-default settings, if any:
* `CREATE TABLE` statements for tables involved:
* Sample data for these tables, use [clickhouse-obfuscator](https://github.com/ClickHouse/ClickHouse/blob/master/programs/obfuscator/Obfuscator.cpp#L42-L80) if necessary

Start by reproducing the issue with the provided YAML variable and SQL examples, then trace how environment variables are handled in dbt-clickhouse. Confirm the fix by verifying that AWS credentials from AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY work without being passed explicitly.

Written by the indexing model from the issue text.