lang/rust: unblock DragonFly build

> rust-stage0-.tar.bz2 doesn't seem to exist in /portdistfiles/.

> Attempting to fetch http://static.rust-lang.org/stage0-snapshots/rust-stage0-.tar.bz2

fetch: http://static.rust-lang.org/stage0-snapshots/rust-stage0-.tar.bz2: Not Found

Reported by: muscles.dragonflybsd.org
Approved by: portmgr "just fix it" blanket