message: [build-utils] Use `File` type for `Prerender` fallback
  Tiny follow-up to since we can now use this union type to express
  the same group of File implementation types.
diff: -import FileBlob from './file-blob';
  -import FileFsRef from './file-fs-ref';
  -import FileRef from './file-ref';
  +import { File } from './types';
  import { Lambda } from './lambda';
  interface PrerenderOptions {
  expiration: number | false;
  lambda: Lambda;
  - fallback: FileBlob | FileFsRef | FileRef | null;
  + fallback: File | null;
  group?: number;
  bypassToken?: string | null /* optional to be non-breaking change */;
  allowQuery?: string[];
  @@ -16,7 +14,7 @@ export class Prerender {
  public type: 'Prerender';
  public expiration: number | false;
  public lambda: Lambda;
  - public fallback: FileBlob | FileFsRef | FileRef | null;
  + public fallback: File | null;
  public group?: number;
  public bypassToken: string | null;
  public allowQuery?: string[];
  