CI: install coverage in the background in all-green

The coverage install sits on the critical path ahead of the artifact
download, which it has nothing to do with, and steps otherwise execute in
sequence. Mark it `background: true` and `wait:` for it once the download
is done. Measured, the install and the download now occupy the same two
second window and the wait costs nothing, taking the job from 20s to 17s.

A background step that fails is reported at the wait, so a broken install
still fails the job.

The alls-green check moves to the end of the job because of an awkward
interaction: a `wait:` step does not accept an `if:` (the workflow fails
to parse with "Unexpected value 'if'"), so the wait cannot carry
`always()` and has to sit in a prefix of steps that nothing failing can
skip. Every step in this job already ran under `always()`, so the check's
position was not gating anything - it only sets the job's conclusion. It
keeps `always()` in its new position so a coverage failure doesn't skip
it and lose the report of which needed job failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GFG654D9KD6rxzQ6R453qm