Deps: update workerpool to fix failing CI tests

[juergba]

Description

Our CI tests are failing with Node v15.8

node:internal/validators:158
      throw new ERR_INVALID_ARG_TYPE(name, 'Object', value);
      ^

TypeError [ERR_INVALID_ARG_TYPE]: The "options" argument must be of type object. Received an instance of Array
    at ChildProcess.target.send (node:internal/child_process:716:7)
    at Array.forEach (<anonymous>)
    at dispatchQueuedRequests (/home/runner/work/mocha/mocha/node_modules/workerpool/src/WorkerHandler.js:262:21)
    at ChildProcess.<anonymous> (/home/runner/work/mocha/mocha/node_modules/workerpool/src/WorkerHandler.js:221:7)
    at ChildProcess.emit (node:events:378:20)
    at emit (node:internal/child_process:920:12)
    at processTicksAndRejections (node:internal/process/task_queues:82:21) {
  code: 'ERR_INVALID_ARG_TYPE'
}

Description of the Change

Upgrade workerpool to v6.1.0