[GH] Switch to using labels to retrigger a workflow.
[GH] Switch to using labels to retrigger a workflow.

* `pull_request_review_comment`s happen a lot more frequently than labelling a PR.
* This method is easier than having to write an action that invokes a `bot`.

Test: Workflows run on GitHub.

This is an imported pull request from https://github.com/androidx/androidx/pull/64.

Resolves #64
Github-Pr-Head-Sha: ee1543adaa0329211009a34407ef3b7872cdc27e
GitOrigin-RevId: ff6a47575d0b7bfdde191b9b33f5d9468a166bd9
Change-Id: I35801979c072fddcfb57dcec074157aac9dab775

AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx