Use tag? instead of tag to indicate that this is boolean

app/models/ci/build.rb

@@ -261,7 +261,7 @@ module Ci
     def predefined_variables
       variables = []
-      variables << { key: :CI_BUILD_TAG, value: ref, public: true } if tag
+      variables << { key: :CI_BUILD_TAG, value: ref, public: true } if tag?
       variables << { key: :CI_BUILD_NAME, value: name, public: true }
       variables << { key: :CI_BUILD_STAGE, value: stage, public: true }
       variables << { key: :CI_BUILD_TRIGGERED, value: 'true', public: true } if trigger_request