message: [build-utils] Remove `as` cast in `Lambda`
  `Files` can now be properly narrowed by checking "type", so no need for the `as` cast here anymore.
diff: @@ -3,10 +3,9 @@ import Sema from 'async-sema';
  import { ZipFile } from 'yazl';
  import minimatch from 'minimatch';
  import { readlink } from 'fs-extra';
  -import { Files, Config } from './types';
  -import FileFsRef from './file-fs-ref';
  import { isSymbolicLink } from './fs/download';
  import streamToBuffer from './fs/stream-to-buffer';
  +import type { Files, Config } from './types';
  interface Environment {
  [key: string]: string;
  @@ -163,7 +162,7 @@ export async function createZip(files: Files): Promise<Buffer> {
  for (const name of names) {
  const file = files[name];
  if (file.mode && isSymbolicLink(file.mode) && file.type === 'FileFsRef') {
  - const symlinkTarget = await readlink((file as FileFsRef).fsPath);
  + const symlinkTarget = await readlink(file.fsPath);
  symlinkTargets.set(name, symlinkTarget);
  }
  }
  @@ -177,7 +176,7 @@ export async function createZip(files: Files): Promise<Buffer> {
  if (typeof symlinkTarget === 'string') {
  zipFile.addBuffer(Buffer.from(symlinkTarget, 'utf8'), name, opts);
  } else {
  - const stream = file.toStream() as import('stream').Readable;
  + const stream = file.toStream();
  stream.on('error', reject);
  zipFile.addReadStream(stream, name, opts);
  }
  