cmake: Only use NO_CMAKE_SYSTEM_PATH when needed for Qt

As-is causes issues with building yuzu using MinGW GCC on Linux-based
machines. Only set the variable when needed. (I'm not quite sure how
this was working before.)

CMakeLists.txt

@@ -337,6 +337,8 @@ if(ENABLE_QT)
         endif()
     endif()
 
+    set(YUZU_QT_NO_CMAKE_SYSTEM_PATH)
+
     # Workaround for an issue where conan tries to build Qt from scratch instead of download prebuilt binaries
     set(QT_PREFIX_HINT)
 
@@ -354,8 +356,10 @@ if(ENABLE_QT)
         endif()
 
         set(QT_PREFIX_HINT HINTS "${QT_PREFIX}")
+
+        set(YUZU_QT_NO_CMAKE_SYSTEM_PATH "NO_CMAKE_SYSTEM_PATH")
     endif()
-    find_package(Qt5 ${QT_VERSION} REQUIRED COMPONENTS Widgets ${QT_PREFIX_HINT} NO_CMAKE_SYSTEM_PATH)
+    find_package(Qt5 ${QT_VERSION} REQUIRED COMPONENTS Widgets ${QT_PREFIX_HINT} ${YUZU_QT_NO_CMAKE_SYSTEM_PATH})
     if (YUZU_USE_QT_WEB_ENGINE)
         find_package(Qt5 COMPONENTS WebEngineCore WebEngineWidgets)
     endif()