Fix Nix build features and remove unused just

[mveril]

Summary

• Stop enabling all Cargo features in the Nix package build, so bench-support remains opt-in.
• Remove the unused just package from the Rust development shells and their package documentation.

Verification

• nixfmt --check flake.nix
• mdbook build docs
• mdbook test docs
• pytest tools/test_book_links.py
• python tools/check-book-links.py docs/book --config docs/book.toml
• nix build .#default (running)