Upgrade attohttpc to 0.29.1 to fix proxy authentication

Volta previously failed to download Node.js through authenticated proxies,
returning "401 Unauthorized" errors. The issue was that attohttpc 0.28
doesn't extract credentials from proxy URLs or send Proxy-Authorization
headers during CONNECT handshakes.

Version 0.29.1 adds proxy authentication support (attohttpc PR #185),
properly handling credentials in proxy URLs like http://user:pass@proxy:port.

Pinned to exactly 0.29.1 because versions 0.29.2+ contain lifetime errors
that don't compile with Rust 1.75 (the project's toolchain version).

Fixes #2090

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: wagenet