Sets the current default JNI env (use with jvm=)

Property definitions

java :: ffi_support $ Sys :: jni_env=
	# Sets the current default JNI env (use with `jvm=`)
	fun jni_env=(jni_env: JniEnv) do jni_env_cache = jni_env
lib/java/ffi_support.nit:52,2--53,57