code: function guessUserName(repo) {
    // extract the host
    const host = repo.match(/:\/\/([^/]+)/)[1];
    const hostClean = host.replace(/\./g, '_').toUpperCase();
    // e.g. GITHUB_COM_CREDENTIALS
    const envVar = process.env[`${hostClean}_CREDENTIALS`];
    if (envVar) {
      return envVar;
    }
    return process.env.PHOVEA_GITHUB_CREDENTIALS;
  }
docs: guesses the credentials environment variable based on the given repository hostname
  @param {string} repo