Disable validation-test/BuildSystem/llvm-targets-options.test on Linux

This has failed a couple of times in CI with an error about cmake not
being an executable. Disbale until we can fix it.

rdar://78320684

Author: benlangmuir

validation-test/BuildSystem/llvm-targets-options.test

@@ -1,3 +1,6 @@
+# Failing in CI with "%cmake is not an executable" rdar://78320684
+# UNSUPPORTED: OS=linux-gnu
+
 # REQUIRES: standalone_build
 
 # RUN: %empty-directory(%t)