Use 666 rather than 644 for default permissions (#5498)
## Summary
I don't know why I used 644 here. 666 is the actual default:
https://github.com/rust-lang/rust/blob/7c2012d0ec3aae89fefc40e5d6b317a0949cda36/library/std/src/sys/pal/unix/fs.rs#L1069
Closes https://github.com/astral-sh/uv/issues/5496.
