When env vars have different value depending on case, throw error

[nedtwigg]

This has the confusing behavior that variables with a lowercase key trump an uppercase key

selfie/jvm/selfie-runner-junit5/src/main/kotlin/com/diffplug/selfie/junit5/SelfieSettingsAPI.kt

Lines 31 to 38 in e6fb9e6

	internal fun calcMode(): Mode {
	val override = lowercaseFromEnvOrSys("selfie") ?: lowercaseFromEnvOrSys("SELFIE")
	if (override != null) {
	return Mode.valueOf(override)
	}
	val ci = lowercaseFromEnvOrSys("ci") ?: lowercaseFromEnvOrSys("CI")
	return if (ci == "true") Mode.readonly else Mode.interactive
	}

I think it's good to be case-insensitive, but if both variables exist and disagree it should generate an easy-to-understand error.