Allow installing dependencies for projects using uv.lock #6
Description
To facilitate transition from pip-tools to a (full) uv environment, the step should check for the presence of a uv.lock and use this for installing the locked dependencies rather than the requirements/ci.txt. To support both old-and-new in parallel, we'll treat the presence of uv.lock as taking precedence over the presence of requirements/ci.txt, and report an error of both are being used simultaneously.
Because uv does not natively/easily allow for installing packages system-wide, we'll use uv export to be able to install with uv pip install --system as we currently do.