Fix uv_build wheel hashes (#15400)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Currently record hashes are the hex encoded sha-256 sum. However,
they're supposed to be urlsafe-base64-nopad.
https://packaging.python.org/en/latest/specifications/recording-installed-packages/#the-record-file
Fixes #15398
## Test Plan
<!-- How was it tested? -->
Build any wheel
```
uv build --wheel
```
Unpack the wheel
```
uvx wheel unpack dist/*.whl
```
Before this change, it will fail with a hash mismatch. I could confirm
with a local build that now the wheel can be unpacked with the `wheel`
command. While I don't enable hash checking when syncing, presumably it
would also currently fail.
