integration_test: only run on system Ruby.

There appear to be random, seemingly impossible to debug issues with
running integration tests on portable Ruby. Instead of confusing
contributors when these will be run on CI anyway: let's just skip them
by default (like we do with `--online` for online tests anyway).

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -9,6 +9,11 @@ RSpec::Matchers.define_negated_matcher :be_a_failure, :be_a_success
 RSpec.shared_context "integration test" do
   extend RSpec::Matchers::DSL
 
+  if OS.mac? &&
+     !RUBY_BIN.to_s.match?(%r{^/(System/Library/Frameworks/Ruby\.framework/Versions/(Current|\d+\.\d+)/)usr/bin$})
+    skip "integration test requires system Ruby"
+  end
+
   matcher :be_a_success do
     match do |actual|
       status = actual.is_a?(Proc) ? actual.call : actual