Disallow `before_script` and `after_script` when `run` is used

The plan for CI Steps is to replace before_script and after_script with setup and teardown respectively. To that end, when run is specified the rails backend currently omits before_script and after_script from the JobRequest. That's OK. However, it is currently possible to create a valid CI job configuration WITH before_script and after_script sections. For example, this CI job is valid and runs:

variables:
  FF_USE_NATIVE_STEPS: true
  STEP_RUNNER_IMAGE: ""registry.gitlab.com/gitlab-org/step-runner/pipelines"
  STEP_RUNNER_VERSION: "982"

stage: test
image:
	name: alpine:latest
before_script: apk add bash             <-- never makes it to the runner
after_script: echo "we are done here"	<-- never makes it to the runner
run:
 - name: the_step
   script: echo "doin' some work..."

The job fails however because the script step requires bash, but it is not installed by default in a vanilla alpine image, and the before_script was (obviously) not run.

This is confusing. This job should produce a syntax error clearly stating that before_script and after_script are not allowed when run is used.