rustbuild: fix local_rebuild

If we detect a local rebuild (e.g. bootstrap compiler is the same version as target compiler), we set stage to 1.
When trying to build e.g. UnstableBook, we use Mode::ToolBootstrap and stage is 1.
Just allow Mode::ToolBootstrap and stagge != 0 if we are in a local_rebuild

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

src/bootstrap/builder.rs

@@ -777,7 +777,7 @@ impl<'a> Builder<'a> {
         // compiler, but for tools we just use the precompiled libraries that
         // we've downloaded
         let use_snapshot = mode == Mode::ToolBootstrap;
-        assert!(!use_snapshot || stage == 0);
+        assert!(!use_snapshot || stage == 0 || self.local_rebuild);
 
         let maybe_sysroot = self.sysroot(compiler);
         let sysroot = if use_snapshot {