Disable UBSan, Clang is not available in upb's Kokoro image atm.

kokoro/ubuntu/build.sh

@@ -22,5 +22,7 @@ if [[ $(uname) = "Linux" ]]; then
 
   # Verify the UBSan build. Have to exclude Lua as the version we are using
   # fails some UBSan tests.
-  CC=clang CXX=clang++ bazel test -c dbg --copt=-fsanitize=undefined --copt=-fno-sanitize=function,vptr --linkopt=-fsanitize=undefined --action_env=UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 -- :all -:test_lua
+
+  # For some reason kokoro doesn't have Clang available right now.
+  #CC=clang CXX=clang++ bazel test -c dbg --copt=-fsanitize=undefined --copt=-fno-sanitize=function,vptr --linkopt=-fsanitize=undefined --action_env=UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1 -- :all -:test_lua
 fi