Fixes https://bugs.dolphin-emu.org/issues/12245.
I considered making a change to DolphinQt instead of
the core, but then additional effort would've been
required to add the same fix to the Android GUI once
we start using the new config system there.
The path to the MoltenVK library can be specified by the
LIBMOLTENVK_PATH environment variable, otherwise it assumes it is
located in the application bundle's Contents/MacOS directory.
Hopefully will fix the crash in vkDestroyInstance on the NV Shield TV,
and likely reduce boot times slightly for drivers that take a while
to create instances.
JosJuice