add test for env mod through system call

Library/Homebrew/rubocops/lines_cop.rb

@@ -135,6 +135,7 @@ module RuboCop
               problem "\"\#\{prefix}#{match[1]}\" should be \"\#{#{match[2].downcase}}\""
             end
           end
+
           find_every_method_call_by_name(body_node, :depends_on).each do |m|
             key, value = destructure_hash(parameters(m).first)
             next if (key.nil? || value.nil?)
@@ -142,11 +143,11 @@ module RuboCop
             problem "#{match[1]} modules should be vendored rather than use deprecated #{m.source}`"
           end
 
-          # find_every_method_call_by_name(body_node, :system).each do |m|
-          #   next unless match = regex_match_group(parameters(m).first, %r{(env|export)(\s+)?})
-          #   problem "Use ENV instead of invoking '#{match[1]}' to modify the environment"
-          # end
-          #
+          find_every_method_call_by_name(body_node, :system).each do |m|
+            next unless match = regex_match_group(parameters(m).first, %r{(env|export)(\s+)?})
+            problem "Use ENV instead of invoking '#{match[1]}' to modify the environment"
+          end
+
           # find_every_method_call_by_name(body_node, :depends_on).each do |m|
           #   next unless modifier?(m)
           #   dep, option = hash_dep(m)

Library/Homebrew/test/rubocops/lines_cop_spec.rb

@@ -1073,6 +1073,28 @@ describe RuboCop::Cop::FormulaAudit::Miscellaneous do
       end
     end
 
+    it "with setting env manually" do
+      source = <<-EOS.undent
+        class Foo < Formula
+          desc "foo"
+          url 'http://example.com/foo-1.0.tgz'
+          system "export", "var=value"
+        end
+      EOS
+
+      expected_offenses = [{  message: "Use ENV instead of invoking 'export' to modify the environment",
+                              severity: :convention,
+                              line: 4,
+                              column: 10,
+                              source: source }]
+
+      inspect_source(cop, source)
+
+      expected_offenses.zip(cop.offenses).each do |expected, actual|
+        expect_offense(expected, actual)
+      end
+    end
+
   end
   def expect_offense(expected, actual)
     expect(actual.message).to eq(expected[:message])