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/tools/compiletest/src/main.rs

@@ -12,7 +12,6 @@
 
 #![feature(box_syntax)]
 #![feature(rustc_private)]
-#![feature(static_in_const)]
 #![feature(test)]
 #![feature(libc)]