rustbuild: Fail the build if we build Cargo twice

This commit updates the `ToolBuild` step to stream Cargo's JSON messages, parse
them, and record all libraries built. If we build anything twice (aka Cargo)
it'll most likely happen due to dependencies being recompiled which is caught by
this check.

src/bootstrap/lib.rs

@@ -254,6 +254,10 @@ pub struct Build {
     ci_env: CiEnv,
     delayed_failures: RefCell<Vec<String>>,
     prerelease_version: Cell<Option<u32>>,
+    tool_artifacts: RefCell<HashMap<
+        Interned<String>,
+        HashMap<String, (&'static str, PathBuf, Vec<String>)>
+    >>,
 }
 
 #[derive(Debug)]
@@ -353,6 +357,7 @@ impl Build {
             ci_env: CiEnv::current(),
             delayed_failures: RefCell::new(Vec::new()),
             prerelease_version: Cell::new(None),
+            tool_artifacts: Default::default(),
         }
     }