Rubocop fix: Perfomance/UnfreezeString (#26217)

lib/tasks/mastodon.rake

@@ -438,12 +438,7 @@ namespace :mastodon do
           "#{key}=#{escaped}"
         end.join("\n")
 
-        generated_header = "# Generated with mastodon:setup on #{Time.now.utc}\n\n".dup
-
-        if incompatible_syntax
-          generated_header << "# Some variables in this file will be interpreted differently whether you are\n"
-          generated_header << "# using docker-compose or not.\n\n"
-        end
+        generated_header = generate_header(incompatible_syntax)
 
         Rails.root.join('.env.production').write("#{generated_header}#{env_contents}\n")
 
@@ -538,6 +533,19 @@ namespace :mastodon do
       puts "VAPID_PUBLIC_KEY=#{vapid_key.public_key}"
     end
   end
+
+  private
+
+  def generate_header(include_warning)
+    default_message = "# Generated with mastodon:setup on #{Time.now.utc}\n\n"
+
+    default_message.tap do |string|
+      if include_warning
+        string << "# Some variables in this file will be interpreted differently whether you are\n"
+        string << "# using docker-compose or not.\n\n"
+      end
+    end
+  end
 end
 
 def disable_log_stdout!