Fix intermittent macOS GHA build issues w/AVX2 (#482)

* Try dropping `CC` on macOS GHA builds

Ideally this specification is not needed to pick up the right compiler
on macOS. So try dropping it to see if things still work.

* Disable AVX2 on macOS

Lately there have been intermittent issues on macOS GHA building with
AVX2 support on. So just turn it off. Hopefully this clears those
errors.

Author: jakirkham

.github/workflows/ci-osx.yaml

@@ -49,7 +49,7 @@ jobs:
         shell: "bash -l {0}"
         run: |
           conda activate env
-          export CC=clang
+          export DISABLE_NUMCODECS_AVX2=""
           python -m pip install -v -e .[test,msgpack,zfpy]
 
       - name: List installed packages