Invoke step-runner from $PATH instead of /

Over in Put step-runner binary in PATH in container (step-runner!138 - merged) we made an alias from /step-runner to /usr/bin/step-runner in the step-runner image. As the MR mentions, this is because over in Native Step Runner Integration for Docker Executor (!5069 - merged) we state that step-runner must be in $PATH for runner steps integration to work. To make that true we implemented Put step-runner binary in PATH in container (step-runner!138 - merged). Now that step-runner is in $PATH, we can update the invocation here.

But why do we want to do this? The other way users can use step-runner is to create their own build image that includes the step-runner binary. In this case, users are more likely to put the binary somewhere in $PATH than at /. And if they did do, the current shim approach to using steps would not work.