Use python 3.11 and the correct conda env on MacOS runners (#1285)

Summary:
While working on https://github.com/pytorch/executorch/pull/1247, I discover an issue where MacOS jobs don't use the correct temporary conda environment and try to install packages directly into the runner base conda setup.  Because the base env is persisted across jobs, this could lead to cryptic issues elsewhere like mismatched PyTorch version.  Also, using a temp environment allows MacOS jobs to use any python it needs instead of the default 3.9 from the base env.

This fixes the root cause of some MacOS flaky issues:
* https://github.com/pytorch/executorch/actions/runs/7000671520/job/19041660402
* https://github.com/pytorch/executorch/actions/runs/6962965183/job/18947693161

Note that `$CONDA_RUN` is a preset command to make sure that the temporary env is used. For example, in https://github.com/pytorch/executorch/actions/runs/7002529232/job/19046476459, `$CONDA_RUN` is set [here](https://github.com/pytorch/executorch/actions/runs/7002529232/job/19046476459#step:4:647)

Pull Request resolved: https://github.com/pytorch/executorch/pull/1285

Reviewed By: digantdesai

Differential Revision: D51582046

Pulled By: huydhn

fbshipit-source-id: c146be51ed6fa0c458cfe80fbb80f3fc800fdc58