fix: shared flock guard for rustup to prevent parallel build races

[AztecBot]

Summary

Noir and avm-transpiler builds run in parallel via Make and both trigger rustup to install/use the Rust toolchain from the shared RUSTUP_HOME at /opt/rust/rustup. When the toolchain isn't cached (e.g. on noir sync branches), both try to install simultaneously, causing:

1. Rustup corruption — concurrent installs race on renaming component files, leaving cargo broken ("the 'cargo' binary is not applicable to the toolchain")
2. Too many open files — parallel cargo builds exhaust file descriptors (addressed separately by the ulimit bump in f4507cc891)

This PR adds a shared flock at /tmp/rustup.lock across all three scripts that interact with rustup:

• noir/bootstrap.sh — wraps install_deps (cargo-binstall, just install-rust-tools)
• avm-transpiler/bootstrap.sh — updates existing local lock (/tmp/rustup-avm-transpiler.lock) to shared path
• bootstrap.sh — wraps the CI-only rustup toolchain install in check_toolchains

The build image Dockerfile already installs Rust 1.89.0 correctly — this guard prevents races when the toolchain needs runtime installation or when rustup operations overlap.

Context: #20985

ClaudeBox log: http://ci.aztec-labs.com/a103561d07144777-2