Disable failing specs on macOS in RubyCI

* https://logs.rubyci.org/osx1400arm/ruby-master/log/20260807T174503Z.fail.html.gz
  1)
  File.setgid? accepts a path in a non-UTF-8, ASCII-compatible encoding containing non-ASCII characters FAILED
  Expected false == true
  to be truthy but was false
  /Users/hsbt/chkbuild/tmp/build/20260807T174503Z/ruby/spec/ruby/shared/file/setgid.rb:11:in 'block (3 levels) in <top (required)>'
  /Users/hsbt/chkbuild/tmp/build/20260807T174503Z/ruby/spec/ruby/core/file/setgid_spec.rb:4:in '<top (required)>'

  2)
  File::Stat#setgid? accepts a path in a non-UTF-8, ASCII-compatible encoding containing non-ASCII characters FAILED
  Expected false == true
  to be truthy but was false
  /Users/hsbt/chkbuild/tmp/build/20260807T174503Z/ruby/spec/ruby/shared/file/setgid.rb:11:in 'block (3 levels) in <top (required)>'
  /Users/hsbt/chkbuild/tmp/build/20260807T174503Z/ruby/spec/ruby/core/file/stat/setgid_spec.rb:5:in '<top (required)>'

  3)
  FileTest.setgid? accepts a path in a non-UTF-8, ASCII-compatible encoding containing non-ASCII characters FAILED
  Expected false == true
  to be truthy but was false
  /Users/hsbt/chkbuild/tmp/build/20260807T174503Z/ruby/spec/ruby/shared/file/setgid.rb:11:in 'block (3 levels) in <top (required)>'
  /Users/hsbt/chkbuild/tmp/build/20260807T174503Z/ruby/spec/ruby/core/filetest/setgid_spec.rb:4:in '<top (required)>'

Author: eregon

shared/file/setgid.rb

@@ -1,5 +1,6 @@
 describe :file_setgid, shared: true do
-  platform_is :darwin do
+  # Fails on RubyCI
+  quarantine! do # platform_is :darwin do
     it "accepts a path in a non-UTF-8, ASCII-compatible encoding containing non-ASCII characters" do
       utf8_path = tmp("file_predicate_utf8_path_\u{3042}.txt")
       # Can fail with UndefinedConversionError if tmp path has non-Shift_JIS chars (e.g. Emojis, Hangul, Cyrillic, accented letters)