Add arm64 URL for MacOS runner to Runner installation modal in CI/CD Runner Settings

What does this MR do and why?

The modal containing installation instructions for runners on MacOS only contains the amd64 URL. This merge request adds the arm64 URL, to bring it inline with the documentation here.

Side note: I am not 100% sure if testing is necessary for this change, nor am I sure how to perform a test beyond whatever CI pipelines creation of this merge request might create if so. Any advice is appreciated. Edit: Tested in GitPod!

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