pyoxidizer: don't use in-memory for resources on macOS
When trying to debug something else on macOS, pyoxidized builds were failing
with the following message about pywatchman (and then cext.base85 when I
commented out pywatchman):
"cannot be loaded from memory but memory loading required"
I don't have any immediate plans to do more work on macOS, but maybe this will
save someone else time debugging this.
Differential Revision: https://phab.mercurial-scm.org/D11451