Modify venv parameter to use dbt executable path instead of venv

[chrishronek]

Currently, we are sourcing the venv here. Instead, we should reference the dbt executable so that implementing the dbt-ol wrapper is easier.

Before:

dbt_path = [bash_path, "-c", f"source {self.python_venv} && dbt {self.base_cmd} "]

After:

{self.python_venv}/bin/dbt {self.base_cmd}