Make os available in cask livecheck blocks

Casks now support an `os` DSL method, similar to `arch`. This makes
it available in `livecheck` blocks, like we do with `arch`.

Library/Homebrew/cask/dsl.rb

@@ -80,6 +80,7 @@ module Cask
       :homepage,
       :language,
       :name,
+      :os,
       :sha256,
       :staged_path,
       :url,

Library/Homebrew/livecheck.rb

@@ -194,9 +194,10 @@ class Livecheck
   end
 
   delegate url_options: :@options
-  delegate version: :@package_or_resource
   delegate arch: :@package_or_resource
-  private :version, :arch
+  delegate os: :@package_or_resource
+  delegate version: :@package_or_resource
+  private :arch, :os, :version
   # Returns a `Hash` of all instance variable values.
   # @return [Hash]
   sig { returns(T::Hash[String, T.untyped]) }

Library/Homebrew/test/livecheck_spec.rb

@@ -182,6 +182,32 @@ RSpec.describe Livecheck do
     end
   end
 
+  describe "#os" do
+    let(:c_os) do
+      Cask::Cask.new("c-os") do
+        os macos: "macos", linux: "linux"
+
+        version "0.0.1"
+
+        url "https://brew.sh/test-0.0.1.dmg"
+        name "Test"
+        desc "Test cask"
+        homepage "https://brew.sh"
+
+        livecheck do
+          url "https://brew.sh/#{os}"
+        end
+      end
+    end
+
+    [:needs_macos, :needs_linux].each do |needs_os|
+      os_value = needs_os.to_s.delete_prefix("needs_")
+      it "delegates `os` in `livecheck` block to `package_or_resource`", needs_os do
+        expect(c_os.livecheck.url).to eq("https://brew.sh/#{os_value}")
+      end
+    end
+  end
+
   describe "#to_hash" do
     it "returns a Hash of all instance variables" do
       expect(livecheck_f.to_hash).to eq(