Collect insns from a child process

to make sure :opt_invokebuiltin_delegate_leave doesn't become
:(trace_)opt_invokebuiltin_delegate.

This is to prevent a warning like
> /tmp/ruby/v3/src/trunk-test/test/ruby/test_jit.rb:618: warning:
'opt_invokebuiltin_delegate_leave' insn is not included in the script.
Actual insns are: opt_invokebuiltin_delegate leave

test/ruby/test_jit.rb

@@ -614,7 +614,10 @@ class TestJIT < Test::Unit::TestCase
 
   def test_compile_insn_opt_invokebuiltin_delegate_leave
     skip 'ld SEGVs for this' if RUBY_PLATFORM.start_with?("s390x-")
-    insns = collect_insns(RubyVM::InstructionSequence.of("\x00".method(:unpack)).to_a)
+    iseq = eval(EnvUtil.invoke_ruby(['-e', <<~'EOS'], '', true).first)
+      p RubyVM::InstructionSequence.of("\x00".method(:unpack)).to_a
+    EOS
+    insns = collect_insns(iseq)
     mark_tested_insn(:opt_invokebuiltin_delegate_leave, used_insns: insns)
     assert_eval_with_jit('print "\x00".unpack("c")', stdout: '[0]', success_count: 1)
   end