kubernetes: Builds fail without `docker buildx`, but dependency isn't validated

What happened:

When I ran make update, it failed with the following error:

ehashman@fedora:~/src/k8s$ make update 
Running in silent mode, run with SILENT=false if you want to see script logs.
Running in short-circuit mode; run with FORCE_ALL=true to force all scripts to run.
Running update-generated-protobuf
+++ Docker build command failed for kube-build:build-09f02a7f40-5-v1.16.5-1

unknown flag: --load
See 'docker --help'.
...

What you expected to happen:

I expected the build to successfully complete, or to be warned that I am missing a required build dependency.

How to reproduce it (as minimally and precisely as possible):

If you don’t have docker buildx installed, you will hit this error. However, that error is masked when you pass the full command line:

ehashman@fedora:~/src/k8s$ docker buildx build
docker: 'buildx' is not a docker command.
See 'docker --help'
ehashman@fedora:~/src/k8s$ docker buildx build --load
unknown flag: --load
See 'docker --help'.

Anything else we need to know?:

Appears to have been introduced in https://github.com/kubernetes/kubernetes/commit/086cdfb3601536f24b127d6e1a16fdd96f468126

Environment:

• Kubernetes version (use kubectl version): HEAD (1.22.x)
• Cloud provider or hardware configuration: N/A, local build
• OS (e.g: cat /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Network plugin and version (if this is a network-related bug):
• Others: