Don’t show Casks on Linux.

Library/Homebrew/cmd/search.rb

@@ -83,7 +83,9 @@ module Homebrew
       remote_formulae = remote_results[:formulae]
       all_formulae = local_formulae + remote_formulae
 
-      local_casks = if query.match?(HOMEBREW_TAP_CASK_REGEX)
+      local_casks = if !OS.mac?
+        []
+      elsif query.match?(HOMEBREW_TAP_CASK_REGEX)
         begin
           [Hbc::CaskLoader.load(query).token]
         rescue Hbc::CaskUnavailableError

Library/Homebrew/test/cmd/search_spec.rb

@@ -25,7 +25,7 @@ describe "brew search", :integration_test do
       .and be_a_success
   end
 
-  it "falls back to a GitHub tap search when no formula is found", :needs_macos, :needs_network, retry: 3 do
+  it "falls back to a GitHub tap search when no formula is found", :needs_network, retry: 3 do
     setup_remote_tap "homebrew/cask"
 
     expect { brew "search", "homebrew/cask/firefox" }