Pin NumPy version on MacOS testers to 1.18.5 (#42409)
Summary:
Otherwise numba linking by clang-9 fails with:
```
ld: in /Users/distiller/workspace/miniconda3/lib/python3.7/site-packages/numpy/core/lib/libnpymath.a(npy_math.o), could not parse object file /Users/distiller/workspace/miniconda3/lib/python3.7/site-packages/numpy/core/lib/libnpymath.a(npy_math.o): 'Unknown attribute kind (61) (Producer: 'LLVM10.0.0' Reader: 'LLVM APPLE_1_902.0.39.2_0')', using libLTO version 'LLVM version 9.1.0, (clang-902.0.39.2)' for architecture x86_64
```
Because conda's numpy-1.19.1 is compiled with clang-10
This should fix MacOS regressions in CIrcleCI
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42409
Reviewed By: xw285cornell
Differential Revision: D22887683
Pulled By: malfet
fbshipit-source-id: d58ee9bf53772b57c59e18f71151916d4f0a3c7d
