Add EE specific VariableEntity logic

app/serializers/variable_entity.rb

 class VariableEntity < Grape::Entity
+  prepend ::EE::VariableEntity
+
   expose :id
   expose :key
   expose :value

ee/app/serializers/ee/variable_entity.rb

+module EE
+  module VariableEntity
+    extend ActiveSupport::Concern
+
+    prepended do
+      expose :environment_scope, if: ->(variable, options) { variable.project.feature_available?(:variable_environment_scope) }
+    end
+  end
+end

spec/ee/spec/serializers/ee/variable_entity_spec.rb

+require 'spec_helper'
+
+describe VariableEntity do
+  let(:variable) { create(:ci_variable) }
+  let(:entity) { described_class.new(variable) }
+
+  describe '#as_json' do
+    subject { entity.as_json }
+
+    context 'when project has variable environment scopes available' do
+      before do
+        allow(variable.project).to receive(:feature_available?).with(:variable_environment_scope).and_return(true)
+      end
+
+      it 'contains the environment_scope field' do
+        expect(subject).to include(:environment_scope)
+      end
+    end
+
+    context 'when project does not have variable environment scopes available' do
+      before do
+        allow(variable.project).to receive(:feature_available?).with(:variable_environment_scope).and_return(false)
+      end
+
+      it 'does not contain the environment_scope field' do
+        expect(subject).not_to include(:environment_scope)
+      end
+    end
+  end
+end