candidates.Remove(paramEnvName)
	}

	isSingleTarget := SubSystemsSingleTargets.Contains(subSys)
	if isSingleTarget && len(candidates) > 0 {
		return Errorf("The following environment variables are unknown: %s",
			strings.Join(candidates.ToSlice(), ", "))
	}

	if !isSingleTarget {
		// Validate other env vars for all targets.
		envVars := candidates.ToSlice()