Skip redundant project configuration parsing for `uv run` (#17890)
## Summary
With `PreviewFeature::TargetWorkspaceDiscovery` enabled, `uv run` was
parsing both the current directory’s project and the target script’s
project. Parsing configuration from the current directory is undesirable
in this case, and might fail with an error if the current directory is
inaccessible.
- Fixes #18687.
- Followup to #17423.
## Test Plan
- See #18687.
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Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
