Make the `uv_build` build backend produce `wheel` and `sdist` files with `0o666 & !umask` permissions on `unix` platforms again (#18020)
## Summary
Fix #18004.
#17276 introduced a regression since `NamedTempFile::new_in` defaults to
`0o600` permissions. Fortunately `uv_fs` already has a wrapper for a
`new_in` with "normal" permissions this so this PR uses that.
## Test Plan
Reliable testing would require us to support setting a umask for a test
context process which we currently don't have support for and
implementing it is not trivial so for now I just tested this manually.
To test, you can just use:
```bash
#!/usr/bin/env bash
set -e
tempdir=$(mktemp -d)
trap 'rm -rf "$tempdir"' EXIT
uv init --lib "$tempdir"
cd "$tempdir"
uv build
umask
ls -al dist/
```