ci: switch to mlugg/setup-zig, pin to 0.17.0-dev.56+a8226cd53

- drop goto-bus-stop/setup-zig and home-lang/pantry-setup /
  home-lang/pantry/packages/action for zig installation
- every workflow now uses mlugg/setup-zig@v2 with the explicit dev
  version so builds are reproducible
- pantry's packages/action@main is kept in release.yml only where it
  handles package publishing, not zig setup

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>