.WithEnv(...): Add support for injecting environment variables in a pipeline step.

[kminehart]

This should support both static values:

pipeline.NamedStep("some-step", action).
    WithEnv(map[string]interface{}{
        "key": "static-value",
    })

Or pipeline.StepArguments, that get resolved when the pipeline is executing:

pipeline.NamedSteps("some-step", action).
    WithEnv(map[string]interface{}{
        "key": pipeline.NewStringArgument("some-value"),
    })