Make the step-runner bootstrap command idempotent

The bootstrap command should not return an error if the destination file exists and is identical to the source file. In this case the command should silently do nothing.

I need this to implement injecting the step-runner binary into the build environment. To keep things simple (i.e. keep the number of step-runner specific execution paths to a minimum) I did the former by invoking the step-runner image with the bootstrap command along with the helper container invocation. It turns out the helper container is run multiple times in the course of one job. Without this change invoking the step-runner bootstrap command fails when it doesn't really need to.