Add slackbot to PR label workflow

[seshubaws]

Which issue(s) does this change fix?

The previous slackbot wasn't triggering because it seems there is a github limitation where events triggered inside a workflow do not create new workflow runs https://docs.github.com/en/actions/how-tos/write-workflows/choose-when-workflows-run/trigger-a-workflow

Why is this change necessary?

How does it address the issue?

What side effects does this change have?

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Review the generative AI contribution guidelines
• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Vandita2020]

How did you test this?

[seshubaws]

How did you test this?

I tried testing locally with gitactions library but that test passed didn't catch that this would be triggered by another workflow. I tried testing in my own fork of the repo but that didn't work bc I didn't have the git tokens... not sure if there is a concrete way to test this locally