Add list of runner labels to runner context

[igagis]

Describe the enhancement
Add runner.labels variable to the runner context which will list all the runner labels.

Code Snippet
Runner labels could be accessed from the workflow via ${{ runner.labels }}.

Additional information
This might be useful to avoid some build steps on self-hosted runners with some pre-installed tools which would normally be installed as a build step on other runners.

[za8457]

https://github.com/za8457/za8457.git

[cv3d]

We also need to know the current runner ID.

[enchorb]

+1 for having a runner id variable exposed

Would be super useful for conditional flows depending on the machine ie. if: ${{ runner.id == 'self-hosted' }}

Ref: https://docs.github.com/en/actions/learn-github-actions/contexts#runner-context

[jupe]

Had to create workaround for this; https://github.com/marketplace/actions/runner-label-resolver, but realise that it's not reasonable to give org level access for access-token just to get runner labels... Hopefully we could get this right.

[github-actions]

Thank you for your interest in the runner application and taking the time to provide your valuable feedback. We kindly ask you to redirect this feedback to the GitHub Community Support Forum which our team actively monitors and would be a better place to start a discussion for new feature requests in GitHub Actions. For more information on this policy please read our contribution guidelines. 😃