Use the system's installation of oaknut.

--- a/externals/CMakeLists.txt
+++ b/externals/CMakeLists.txt
@@ -109,7 +109,7 @@ endif()
 
 # Oaknut
 if ("arm64" IN_LIST ARCHITECTURE)
-    add_subdirectory(oaknut EXCLUDE_FROM_ALL)
+    add_library(oaknut INTERFACE)
 endif()
 
 # Dynarmic