Freeze more strings

Freeze the results changed in #6072.

Library/Homebrew/cask/exceptions.rb

@@ -182,7 +182,7 @@ module Cask
         s << "\n" unless reason.end_with?("\n")
       end
 
-      s
+      s.freeze
     end
   end
 
@@ -196,7 +196,7 @@ module Cask
         s << "\n" unless reason.end_with?("\n")
       end
 
-      s
+      s.freeze
     end
   end
 
@@ -210,7 +210,7 @@ module Cask
         s << "\n" unless reason.end_with?("\n")
       end
 
-      s
+      s.freeze
     end
   end
 end

Library/Homebrew/cmd/gist-logs.rb

@@ -90,7 +90,7 @@ module Homebrew
       s << "Host: #{hostname}\n"
     end
     s << "Build date: #{build_time_str}\n"
-    s
+    s.freeze
   end
 
   # Causes some terminals to display secure password entry indicators