Enforce root timeline paths to be absolute

We currently do [such enforcement][1] for the `get_file_metadata` action and the Python agent [does it][2] for the timeline action.

Without it we can leak agent working directory (which is rather harmless but better to avoid this if possible).

[1]: https://github.com/google/rrg/blob/7f8dd150f57dad2f5d97d777e15e4e961f3b7a02/crates/rrg/src/action/get_file_metadata.rs#L74-L79
[2]: https://github.com/google/grr/blob/9491bf2bd7875371fd0d99cc82cf10e072b311b7/grr/client/grr_response_client/client_actions/timeline.py#L75-L76