Fixed build-watch task

[jasonbeverage]

This fixes the build-watch task with the upgraded gulp. Without this change you get:
Error: watching Specs//*.js,!Specs/SpecList.js,Source/Shaders//*.glsl: watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)

[cesium-concierge]

Thanks for the pull request @jasonbeverage!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[mramato]

Thanks @jasonbeverage