Update Cargo to fix nightly channel

This commit updates Cargo with rust-lang/cargo#3820 which includes a fix for
rust-lang/cargo#3819. At the same time this also slightly tweaks how rustbuild
builds cargo to ensure that all the build information (including git info and
such) makes its way into the binary.

Closes rust-lang/cargo#3820

src/bootstrap/lib.rs

@@ -232,7 +232,7 @@ impl Build {
             None => false,
         };
         let rust_info = channel::GitInfo::new(&src);
-        let cargo_info = channel::GitInfo::new(&src.join("src/tools/cargo"));
+        let cargo_info = channel::GitInfo::new(&src.join("cargo"));
 
         Build {
             flags: flags,