Codecov - Alpine image not auto-detected on Action.

## The fix

The codecov-action's platform auto-detection doesn't cover Alpine, so on Alpine-based containers the uploader binary download fails. Fix: point the action at the codecov wrapper download script (the download.sh in codecov/wrapper) instead of relying on auto-detection, and the right binary gets fetched. The maintainer confirmed this approach.

Thread: gh:codecov/codecov-action#1262