fix for local anchors

[randomlogin]

It's impossible to run fabric with local root anchors file:

$ fabric --local-anchors rootanchors.json
Must specify exactly one of local, remote, or static anchors.

It always sets remote anchors even if they were not provided. But later there is a check whether only 1 options was used:

  private constructor(private options: UpdateOptions) {
    const usingLocal = !!options.localPath;
    const usingRemote = !!options.remoteUrls;
    const usingStaticAnchors = !!options.staticAnchors;

    if ([usingLocal, usingRemote, usingStaticAnchors].filter(Boolean).length != 1) {
      throw new Error('Must specify exactly one of local, remote, or static anchors.');
    }

[buffrr]

LGTM thanks