Only install default cask tap automatically on macOS.

Library/Homebrew/cask/cmd.rb

@@ -167,7 +167,7 @@ module Cask
 
       args = self.class.parser.parse(argv, ignore_invalid_options: true)
 
-      Tap.default_cask_tap.install unless Tap.default_cask_tap.installed?
+      Tap.install_default_cask_tap_if_necessary
 
       command, argv = detect_internal_command(*argv) ||
                       detect_external_command(*argv) ||

Library/Homebrew/cli/named_args.rb

@@ -97,11 +97,7 @@ module Homebrew
           begin
             return Cask::CaskLoader.load(name, config: Cask::Config.from_args(@parent))
           rescue Cask::CaskUnavailableError => e
-            default_cask_tap = Tap.default_cask_tap
-            unless default_cask_tap.installed?
-              default_cask_tap.install
-              retry
-            end
+            retry if Tap.install_default_cask_tap_if_necessary
 
             raise e if only == :cask
           end

Library/Homebrew/diagnostic.rb

@@ -560,10 +560,10 @@ module Homebrew
       end
 
       def check_casktap_git_origin
-        cask_tap = Tap.default_cask_tap
-        return unless cask_tap.installed?
+        default_cask_tap = Tap.default_cask_tap
+        return unless default_cask_tap.installed?
 
-        examine_git_origin(cask_tap.path, cask_tap.remote)
+        examine_git_origin(default_cask_tap.path, default_cask_tap.remote)
       end
 
       sig { returns(T.nilable(String)) }
@@ -921,12 +921,12 @@ module Homebrew
       end
 
       def check_cask_taps
-        default_tap = Tap.default_cask_tap
-        alt_taps = Tap.select { |t| t.cask_dir.exist? && t != default_tap }
+        default_cask_tap = Tap.default_cask_tap
+        alt_taps = Tap.select { |t| t.cask_dir.exist? && t != default_cask_tap }
 
         error_tap_paths = []
 
-        add_info "Homebrew Cask Taps:", ([default_tap, *alt_taps].map do |tap|
+        add_info "Homebrew Cask Taps:", ([default_cask_tap, *alt_taps].map do |tap|
           if tap.path.blank?
             none_string
           else

Library/Homebrew/extend/os/mac/tap.rb

@@ -0,0 +1,11 @@
+# typed: true
+# frozen_string_literal: true
+
+class Tap
+  def self.install_default_cask_tap_if_necessary
+    return false if default_cask_tap.installed?
+
+    default_cask_tap.install
+    true
+  end
+end

Library/Homebrew/extend/os/tap.rb

@@ -2,3 +2,4 @@
 # frozen_string_literal: true
 
 require "extend/os/linux/tap" if OS.linux?
+require "extend/os/mac/tap" if OS.mac?

Library/Homebrew/tap.rb

@@ -60,10 +60,16 @@ class Tap
     fetch(match[:user], match[:repo])
   end
 
+  sig { returns(T.attached_class) }
   def self.default_cask_tap
     @default_cask_tap ||= fetch("Homebrew", "cask")
   end
 
+  sig { returns(T::Boolean) }
+  def self.install_default_cask_tap_if_necessary
+    false
+  end
+
   extend Enumerable
 
   # The user name of this {Tap}. Usually, it's the GitHub username of