add builder-config into tarball sources

This will be useful for certain scenarios where developers want to know
how the tarball sources were generated. We also want this to check for CI
rustc incompatible options on bootstrap.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/utils/tarball.rs

@@ -317,6 +317,12 @@ impl<'a> Tarball<'a> {
             channel::write_commit_hash_file(&self.overlay_dir, &info.sha);
             channel::write_commit_info_file(&self.overlay_dir, info);
         }
+
+        // Add config file if present.
+        if let Some(config) = &self.builder.config.config {
+            self.add_renamed_file(config, &self.overlay_dir, "builder-config");
+        }
+
         for file in self.overlay.legal_and_readme() {
             self.builder.install(&self.builder.src.join(file), &self.overlay_dir, 0o644);
         }