fix(config): respect base_dir in keypair generation

[folex]

Description

@nahsi asked:

Can we make nox to put keys to BASE_DIR/... by default instead of /.fluence?

It turns out, the current keypair generation does not respect base_dir property in config. It always uses /.fluence as a base dir, and puts key there.

Proposed Changes

Load and store keypairs relative to base_dir in Nox config.

Checklist

• The code follows the project's coding conventions and style guidelines.
• All tests related to the changes have passed successfully.
• Documentation has been updated to reflect the changes (if applicable).
• All new and existing unit tests have passed.
• I have self-reviewed my code and ensured its quality.
• I have added/updated necessary comments to aid understanding.

Reviewer Checklist

• Code has been reviewed for quality and adherence to guidelines.
• Tests have been reviewed and are sufficient to validate the changes.
• Documentation has been reviewed and is up to date.
• Any questions or concerns have been addressed.