Makes the following changes to the behavior of precache:
- The --all-platforms flags now fetches all artifacts, rather than just
  turning off platform filtering of selected artifacts.
- Explicitly requested artifacts are no longer subject to platform
  filtering. E.g., 'precache --ios' will download iOS artifacts on
  Windows and Linux (but 'precache' without an 'ios' flag will still
  only download iOS artifacts on macOS).
- Desktop platform artifacts now respect the bypassing of platform
  filtering.

Fixes #53272