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,
+ },
 }