Allow package args, dont run post deploy steps if empty

[acbramley]

For the skpr website we need to add --build-arg to pass through an NPM token. Also we don't need post deploy steps.

[acbramley]

I'm hoping this will let us do:

    with:
      env: dev
      suspend_cron: false
      post_deploy: ''
      package_args: --build-arg=NODE_AUTH_TOKEN=${{ env.NODE_AUTH_TOKEN }}