skip the test now we couldn't fix

The following error is reported repeatedly on my riscv64-linux machine, so just skipt it.
I hope someone investigate it.

```
  1) Error:
TestStruct::SubStruct#test_named_structs_are_not_rooted:
Test::Unit::ProxyError: execution of Test::Unit::CoreAssertions#assert_no_memory_leak expired timeout (10 sec)
pid 1113858 killed by SIGTERM (signal 15)
| ruby 3.5.0dev (2025-05-22T21:05:12Z master 9583b7a) +PRISM [riscv64-linux]
|
| [7;1m1096282:1747967727.622:d70f:[mSTART={peak:453828608,size:453763072,lck:0,pin:0,hwm:9601024,rss:9601024,data:445943808,stk:135168,exe:4096,lib:7450624,pte:77824,swap:0}
| [7;1m1096282:1747967727.622:d70f:[mFINAL={peak:457502720,size:457498624,lck:0,pin:0,hwm:13312000,rss:13312000,data:449679360,stk:135168,exe:4096,lib:7450624,pte:86016,swap:0}
```

Author: ko1

test/ruby/test_struct.rb

@@ -535,6 +535,8 @@ def test_parameters
   end
 
   def test_named_structs_are_not_rooted
+    omit 'skip on riscv64-linux CI machine. See https://github.com/ruby/ruby/pull/13422' if ENV['RUBY_DEBUG'] == 'ci' && /riscv64-linux/ =~ RUBY_DESCRIPTION
+
     # [Bug #20311]
     assert_no_memory_leak([], <<~PREP, <<~CODE, rss: true)
       code = proc do