cask/cmd/list_spec.rb: fix leaky version variable

The lazy evaluation in let() was failing to reset MacOS#full_version
to the original_macos_version. This meant that all tests run after
this one automatically had MacOS#version == 12 which caused some
of the tests to fail if you were running a different macOS version.

Library/Homebrew/test/cask/cmd/list_spec.rb

@@ -246,7 +246,7 @@ describe Cask::Cmd::List, :cask do
         ]
       EOS
     }
-    let(:original_macos_version) { MacOS.full_version.to_s }
+    let!(:original_macos_version) { MacOS.full_version.to_s }
 
     before do
       # Use a more limited symbols list to shorten the variations hash