Skip test_ln_sr(FileUtilsSingletonTest) ib rbs tests

ruby/fileutils#139
https://github.com/ruby/actions/actions/runs/16425309325/job/46414287784

Author: hsbt

tool/rbs_skip_tests

@@ -79,3 +79,6 @@ CGISingletonTest CGI is retired
 RactorSingletonTest Ractor API was changed https://bugs.ruby-lang.org/issues/21262
 RactorInstanceTest  Ractor API was changed https://bugs.ruby-lang.org/issues/21262
 
+# https://github.com/ruby/fileutils/pull/139
+# https://github.com/ruby/actions/actions/runs/16425309325/job/46414287784
+test_ln_sr(FileUtilsSingletonTest)