docker buildx create appears to have no driver specific option or mechanism for the Kubernetes driver to specify the service account the deployment should use. As far as I can tell there is no way to set this other than to manually modify the deployment itself after it as been created by applying a patch like this:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: example-buildkit0
  name: example-buildkit0
spec:
  selector:
    matchLabels:
      app: example-buildkit0
  template:
    spec:
      serviceAccountName: buildx-builder
      containers:
        - name: buildkitd
          image: moby/buildkit:buildx-stable-1

It would be highly beneficial to allow the ability to set serviceAccounts on creation as these are often needed to access external resources such as a storage bucket using WorkloadIdentity. This is a fairly fundamental need for builds.