Write relative paths for scripts in data directory (#2348)
## Summary
In #2000, I shipped a regression whereby we stopped writing relative
paths for scripts within `data` directories. The net effect here is that
we aren't _uninstalling_ binaries in all cases. (This does _not_ apply
to entrypoints, only scripts in `data` directories.)
Closes https://github.com/astral-sh/uv/issues/2330.
## Test Plan
Most Python packages ship entrypoints, not binaries, so I don't know how
to test this cheaply. But I did test it locally by verifying that `uv`
is now removed from the `bin` directory after an uninstall.
