build: Try some additional micro-optimizations (#48028)

* The "Prepare" step probably doesn't need php installed.
* If we run into a GitHub issue during the push in the "Build wpcom
  projects" job, a re-run would require a re-build. Set
  continue-on-error for those steps to allow the workflow to proceed
  (and retry the push in the "Push to mirror repos" job).
* We lost the re-ordering from #45763 in the "Push to mirror repos" job
  (not that it should usually matter anymore), since the new merge step
  rewrites the mirrors.txt file. Move the logic for reordering
  mirrors.txt into the "Push to mirror repos" job, just in case of those
  rare retries.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/24210787160

Upstream-Ref: Automattic/jetpack@23c480a

Author: tbradsha