Handle build completion message from Cargo

This was introduced in the recent bump to 1.43 bootstrap cargo

Author: Mark-Simulacrum

src/bootstrap/compile.rs

@@ -1012,4 +1012,7 @@ pub enum CargoMessage<'a> {
     BuildScriptExecuted {
         package_id: Cow<'a, str>,
     },
+    BuildFinished {
+        success: bool,
+    },
 }