Only run Valgrind if it is present.

kokoro/ubuntu/build.sh

@@ -32,5 +32,8 @@ if [[ $(uname) = "Linux" ]]; then
   # 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
+
+if which valgrind; then
   bazel test --run_under='valgrind --leak-check=full --error-exitcode=1' :all -- -:test_conformance_upb -:cmake_build
 fi