Use top-level `--isolated` for `uv run` (#3431)
## Summary
We already have a global `--isolated`, which means "ignore any on-disk
configuration". I think we should reuse this for the "ignore the
workspace" setting in `uv run`, rather than `--no-workspace`.
I've also merged the existing `--isolated` and `--no-workspace`
behaviors in `uv run` into a single flag. We may not need separate flags
for this, since the current intent seems to be "ignore the workspace
environment"? Though we could always re-add later.
Closes https://github.com/astral-sh/uv/issues/3421.
