Do not let the stage0 sysroot copy write through the rustc-src symlink

With rustc-dev or rust-src installed in the stage0 toolchain, the stage0
sysroot already holds symlinks to the source root when its lib directory is
copied again. cp_link_r then replaces files in the source tree with the
stage0 compiler sources, so the vendored crates no longer match the manifests.

--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -818,6 +818,12 @@
                 let _ = fs::remove_dir_all(sysroot.join("lib/rustlib/src/rust"));
             }
 
+            // Sysroot steps symlink these to the source root. Copying the
+            // stage0 lib over an existing symlink writes through it and
+            // replaces the source tree with the stage0 compiler sources.
+            let _ = fs::remove_dir_all(sysroot.join("lib/rustlib/rustc-src/rust"));
+            let _ = fs::remove_dir_all(sysroot.join("lib/rustlib/src/rust"));
+
             builder.cp_link_r(&builder.initial_sysroot.join("lib"), &sysroot.join("lib"));
         } else {
             if builder.download_rustc() {