Rollup merge of rust-lang#52969 - Keruspe:local_rebuild, r=alexcrichton

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

This fixes building current master using current beta (as master hasn't yet been bumped to 1.30).
This should be backported to beta too, as currently we cannot build beta using itself because of that.

r? @alexcrichton

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 {