How can I use a non-exec entrypoint for Kaniko in Google Cloud Build (to enable build arg definition)

The instructions for using Kaniko in GCB use the exec form of the kaniko project builder, like this:

  - id: 'Build (with Kaniko Cache)'
    name: 'gcr.io/kaniko-project/executor:latest'
    args:
      - --destination=$_GCR_HOSTNAME/$PROJECT_ID/$REPO_NAME:$SHORT_SHA
      - --cache=true
      - --cache-ttl=6h

But I'm using it to replace a docker build, in which I circumvent the exec form of usage in order to inject a build arg (an access token from the Secret Manager) as described here and here.

  - id: 'Build'
    name: gcr.io/cloud-builders/docker
    entrypoint: 'bash'
    args:
      - '-c'
      - |
        docker build --cache-from $_GCR_HOSTNAME/$PROJECT_ID/$REPO_NAME:$SHORT_SHA --build-arg PERSONAL_ACCESS_TOKEN_GITHUB=$(cat decrypted-pat.txt) -t $_GCR_HOSTNAME/$PROJECT_ID/$REPO_NAME:$SHORT_SHA .

I've tried defining a bash entrypoint but that's not found so I'm stuck. Is it even possible to run the non-exec form?

Note: It is possible to access the secret in a file within the container instead of via a build arg, but that would mean changing the setup for my developers to all have that secret file in order to build their development images locally, which I could, but really don't want, to do.

I solved it using docker run:

- id: Build
  name: gcr.io/cloud-builders/docker
  entrypoint: /bin/bash
  args:
  - -c
  - |
   docker run \
      --network=cloudbuild \
      -v /workspace:/workspace \
        gcr.io/kaniko-project/executor:latest \
          --dockerfile /workspace/Dockerfile \
          --build-arg=GITHUBTOKEN=$$GITHUBTOKEN \
          --destination=gcr.io/$PROJECT_ID/myapp:$SHORT_SHA \
          --cache=true \
          --context dir:///workspace/
  secretEnv: ['GITHUBTOKEN']

availableSecrets:
  secretManager:
    - versionName: projects/$PROJECT_ID/secrets/github_machine_user_pat/versions/latest
      env: GITHUBTOKEN