Skip a test_jit with builtin for rhel_zlinux

Either 95b0fed371 or 7561db8c00 started to cause
https://rubyci.org/logs/rubyci.s3.amazonaws.com/rhel_zlinux/ruby-master/log/20200621T053303Z.fail.html.gz

But so far no idea why it's happening. Until I get direct ssh access to
debug the details, let me skip this as it's essentially not ruby's fault.

test/ruby/test_jit.rb

@@ -613,6 +613,7 @@ class TestJIT < Test::Unit::TestCase
   end
 
   def test_compile_insn_opt_invokebuiltin_delegate_leave
+    skip 'ld SEGVs for this' if ENV['RUBYCI_NICKNAME'] == 'rhel_zlinux'
     insns = collect_insns(RubyVM::InstructionSequence.of("\x00".method(:unpack)).to_a)
     mark_tested_insn(:opt_invokebuiltin_delegate_leave, used_insns: insns)
     assert_eval_with_jit('print "\x00".unpack("c")', stdout: '[0]', success_count: 1)