Add DRONE_STEP_NUMBER to environment variables

[dnephin]

We've noticed that https://docs.drone.io/pipeline/environment/reference/drone-step-number/ is not being set in the environment variables for pipelines run using this kube runner.

I believe the problem is this:

The pod environment is created in engine.Setup from calls toPod -> toContainers -> toContainer -> toEnv. When this happens we don't yet have a StepNumber, we've only set the DRONE_STEP_NAME.

engine.Setup is called from Execer.Exec, and later in that function in a call to Execer.exec the DRONE_STEP_NUMBER is supposed to set by on the step here. At this point the pod spec was already created, so this env var never makes it to the pod.

In this PR I've attempted to fix the problem by passing a step number in Compiler.Compile where the rest of the environment variables are set. I found that this is how #61 fixed the DRONE_STEP_NAME env variable.

This fix is maybe not ideal because it's assuming the steps are not re-ordered in any way later, but it seems like it may be the only way to fix this problem without a larger change to where the pods specs are created.

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