tapioca/compilers/cask/config: Add Linux-only `Cask::Config` keys too

- The DSL compiler iterates over `Cask::Config.defaults` to emit accessor
  methods. On macOS that map omits `appimagedir` because it lives in the
  Linux-only `OS::Linux::Cask::Config::ClassMethods::DEFAULT_DIRS` (moved
  there in 48ac0fb503). Running `brew tc --update` on macOS therefore
  deleted `def appimagedir; end` from the RBI, which then broke Sorbet's
  view of `cask/artifact/appimage.rb`.

- Maintain a small list of Linux-only dir names in the compiler and merge
  them into the key set so the generated RBI is OS-independent. Trying
  to retrieve these dynamically failed on macOS because the Linux-only
  defaults are inaccessible.

Co-Authored-By: Copilot (CLI, agent mode, model Claude Opus 3.7)