rustbuild doesn't support DESTDIR

[ncihnegn]

No destdir is passed to install.sh in install.rs.

[Randl]

That causes fail in Gentoo build gentoo/gentoo-rust#217

[Randl]

As far as I understand, we need to to pass DESTDIR to install function and add value of DESTDIR to prefix here https://github.com/rust-lang/rust/blob/master/src/bootstrap/install.rs#L27