message: [build-utils] Use `File` instead of `FileBase` for builder output type
  This allows for the individual `File` subtypes to be properly narrowed.
diff: @@ -360,7 +360,7 @@ export interface BuildResultV2 {
  routes?: any[];
  images?: Images;
  output: {
  - [key: string]: FileBase | Lambda | Prerender | EdgeFunction;
  + [key: string]: File | Lambda | Prerender | EdgeFunction;
  };
  wildcard?: Array<{
  domain: string;
  