USE_SYSTEM_OPENXR=ON proton fails with "Loader does not support simultaneous XrInstances"

Proton probes for OpenVR and OpenXR support at startup, as seen in Monado logs:

 INFO [client_loop] Client 1 connected
 INFO [ipc_handle_instance_describe_client] Client info:
	id: 1
	application_name: 'OpenComposite_wine64-preloader'
	pid: 539772
	extensions:
		ext_hand_tracking_enabled: true
		ext_eye_gaze_interaction_enabled: false
		ext_hand_interaction_enabled: false
 INFO [client_loop] Client 2 connected
 INFO [ipc_handle_instance_describe_client] Client info:
	id: 2
	application_name: 'wineopenxr test instance'
	pid: 539772
	extensions:
		ext_hand_tracking_enabled: false
		ext_eye_gaze_interaction_enabled: false
		ext_hand_interaction_enabled: false
 INFO [client_loop] Client disconnected.
 INFO [client_loop] Client disconnected.

Both wineopenxr and OpenComposite will attempt to create an OpenXR instance, and as seen in logs instances are alive concurrently.

OpenXR loader has a check for simultaneous instances in loader_core.cpp. The check is backed by a global variable in loader_instance.cpp.

When setting USE_SYSTEM_OPENXR=OFF, OpenComposite will have its private copy of the global variable, and check passes. When USE_SYSTEM_OPENXR=ON, both load the same shared library and fail.