Update Rust toolchain to nightly-2026-07-29 (#96365)
This updates to nightly-2026-07-29
---------
Co-authored-by: vercel-fleet[bot] <308483924+vercel-fleet[bot]@users.noreply.github.com>
Co-authored-by: Will Binns-Smith <755844+wbinnssmith@users.noreply.github.com>
