rustbuild: Update bootstrap compiler

Now that we've also updated cargo's release process this commit also changes the
download location of Cargo from Cargos archives back to the static.r-l.o
archives. This should ensure that the Cargo download is the exact Cargo paired
with the rustc that we release.

src/librustc_data_structures/lib.rs

@@ -27,7 +27,6 @@
 
 #![feature(shared)]
 #![feature(collections_range)]
-#![cfg_attr(stage0,feature(field_init_shorthand))]
 #![feature(nonzero)]
 #![feature(rustc_private)]
 #![feature(staged_api)]