Merge changes from topics "rulebuilder-ninja-vars", "rustc_prebuilt_build_tool" into main

* changes:
  allow Ninja variables in RuleBuilder API
  add crate_root property to rust modules
  add rust_toolchain_rustc_prebuilt module type

cmd/sbox/sbox.go

@@ -119,6 +119,9 @@ func run() error {
 	}
 
 	manifest, err := readManifest(manifestFile)
+	if err != nil {
+		return err
+	}
 
 	if len(manifest.Commands) == 0 {
 		return fmt.Errorf("at least one commands entry is required in %q", manifestFile)