tolerate when bazel shutdown errors out

pull-kubernetes-e2e-gce-rbe are still failing with the following:
```
INFO: Waited 10 seconds for server process (pid=72) to terminate.
FATAL: Attempted to kill stale server process (pid=72) using SIGKILL, but it did not die in a timely fashion.
make: *** [Makefile:626: bazel-release] Error 36
make: Leaving directory '/home/prow/go/src/k8s.io/kubernetes'
```

we have added a pkill just after the line for bazel shutdown, so let's
continue to give the pkill a chance to run.

Author: dims

build/root/Makefile

@@ -623,6 +623,6 @@ bazel-release:
 else
 bazel-release:
 	bazel build //build/release-tars
-	bazel shutdown
+	bazel shutdown || true
 	pkill ^bazel || true
 endif