Only add step-runner volume mount when native steps is enabled

Over in Inject the step-runner binary into the build co... (!5322 - merged) we inject the step-runner binary into the build container. To do this we mount a common volume on all 3 (build, helper, step-runner containers).

Turns out the container mount point is not valid on windows. However:

• We should only really add this column mount when native steps is enabled.
• We should not do this on windows since step-runner hasn't even been tested on windows.
• We don't need to mount this volume on the helper container anyway.

For now let's only mount it when native steps is enabled. We'll figure out the windows story later, and the helper image will disappear (hopefully) soon anyway.

Closes Windows gitlab-runner-helper image fails due to... (#38632 - closed)