Use python 3.9 for iOS build and test (#91366)
Since yesterday, Miniconda3-latest-MacOSX-x86_64.sh has changed to python 3.10 as the default, and it breaks iOS workflow:
* Breaking with python 3.10 https://github.com/pytorch/pytorch/actions/runs/3763269382/jobs/6396697341
* Working with python 3.9 https://github.com/pytorch/pytorch/actions/runs/3761903011/jobs/6394085845
Fun fact, both examples above come from the same commit https://hud.pytorch.org/pytorch/pytorch/commit/f471770fd40cae2065a8d066932ddf59e12a758d (one was in periodic, the other was in trunk)
Miniconda3-py39_4.12.0-MacOSX-x86_64.sh is the same miniconda installation that we use in https://github.com/pytorch/test-infra/tree/main/.github/actions/setup-miniconda
Note: @remidomingues is trying to add cache support for iOS in on https://github.com/pytorch/pytorch/pull/91359. The PR is still under review. But once that is merged, this issue won't happen again. So this is a temporary fix to keep trunk green.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91366
Approved by: https://github.com/atalman
