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 48ac0fb). 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)

Author: issyl0

Library/Homebrew/sorbet/rbi/dsl/cask/config.rbi

@@ -9,22 +9,29 @@ module Compilers
     class CaskConfig < Tapioca::Dsl::Compiler
       ConstantType = type_member { { fixed: T::Module[T.anything] } }
 
+      # Dirs defined in `OS::Linux::Cask::Config::ClassMethods::DEFAULT_DIRS`
+      # that aren't visible to `Cask::Config.defaults` when this compiler is
+      # run on macOS, but still need accessor methods generated in the RBI.
+      LINUX_ONLY_DIRS = T.let([:appimagedir].freeze, T::Array[Symbol])
+
       sig { override.returns(T::Enumerable[T::Module[T.anything]]) }
       def self.gather_constants = [Cask::Config]
 
       sig { override.void }
       def decorate
+        keys = Cask::Config.defaults.keys | LINUX_ONLY_DIRS
+
         root.create_module("Cask") do |mod|
           mod.create_class("Config") do |klass|
-            Cask::Config.defaults.each do |key, value|
+            keys.each do |key|
               return_type = if key == :languages
                 # :languages is a `LazyObject`, so it lazily evaluates to an
                 # array of strings when a method is called on it.
                 "T::Array[String]"
               elsif key.end_with?("?")
                 "T::Boolean"
               else
-                value.class.to_s
+                "String"
               end
 
               klass.create_method(key.to_s, return_type:, class_method: false)