Avoid hardcoding file paths in _is_local_manifest_command

In the `_is_local_manifest_command` function, the path to the `manifest.yaml` is hardcoded as `"/airbyte/integration_code/source_declarative_manifest/manifest.yaml"`. It would be beneficial to derive this path dynamically or make it configurable to improve portability, especially if the directory structure changes in the future.

**Reference:**
- PR: https://github.com/airbytehq/airbyte-python-cdk/pull/58
- Comment: https://github.com/airbytehq/airbyte-python-cdk/pull/58#discussion_r1842974718

Requested by: @aaronsteers