The string `msl_patch_default` can have been read
partially uninitialized or initialized multiple
time and read uncomplete during multithreaded
compilation.
This should fix the GPU tests randomly failing on mac.
While this would never fail when blender runs from the UI (since
UI shaders are init in single threaded manner and always compile
before EEVEE shaders), this race condition could happen when running
EEVEE through background rendering or running tests.
Pull Request: https://projects.blender.org/blender/blender/pulls/131580
7b6cc57215