[XEB] Multiprocessing spawn (#4090)

There are two multiprocessing "modes": fork and spawn. Fork used to always be the default. Now sometimes on Mac it defaults to spawn (?) but in any event: spawn works way better. This could prevent some of the c++-looking errors that @mrwojtek has seen. It fixed different-but-still-c++-looking-errors for me

cirq-google/cirq_google/calibration/xeb_wrapper.py

@@ -38,7 +38,7 @@ def _maybe_multiprocessing_pool(
         yield None
         return
 
-    with multiprocessing.Pool(processes=n_processes) as pool:
+    with multiprocessing.get_context('spawn').Pool(processes=n_processes) as pool:
         yield pool