Fallback to interpreter discovery in `uv run` (#4549)
## Summary
This PR modifies `uv run` to fallback to discovering an interpreter
(e.g., a local `.venv`) if the command is run outside of a workspace.
`uv run --isolated` continues to completely skip workspace _and_
interpreter discovering, only installing whatever's provided with
`--with`.
The next step here is adding some ergonomic controls for enabling this
behavior even if your project is technically in a workspace (i.e., you
have a `pyproject.toml` but aren't using the Project APIs and don't want
locking etc.). I could imagine a setting in `pyproject.toml` that's also
exposed on the command-line. Something like: `managed = false` or
`project = false`.
See: https://github.com/astral-sh/uv/issues/3836.
