Rename runner,finalize stages to better match contents

What does this MR do?

Since !109 (merged) the runner stage only contains Xcode installation steps, and the finalize stage installs the runner. Renames runner to xcode and finalize to runner to better match this. Individual commits have been created for each rename, to ease review.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?