Fix CI job (#73)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/73

Apparently in macos CI, python3 and python are pointing to different python environment.

python3 -> /Library/Python/3.9/site-packages/

python -> /Users/ec2-user/runner/_work/_temp/miniconda/lib/python3.9/site-packages

This diff is fixing the CI failure:
```
[2023-08-17T02:59:12.739+00:00] Local command: env -- "ASAN_OPTIONS=detect_leaks=0,detect_odr_violation=0" "GEN_DIR=GEN_DIR_DEPRECATED" "OUT=././../out" "SRCDIR=./." "SRCS=././link_torch.sh" /usr/bin/env bash -e buck-out/v2/gen/root/213ed1b7ab869379/third-party/__libtorch_gen__/sh/genrule.sh
[2023-08-17T02:59:12.739+00:00] Stdout: Error: /Library/Python/3.9/site-packages/torch/lib/libtorch.dylib doesn't exist
```

Also adding `tomli` as a ci requirements due to the following error:
```
Error:
Traceback (most recent call last):
  File "/Users/ec2-user/runner/_work/executorch/executorch/pytorch/executorch/build/extract_sources.py", line 18, in <module>
    import tomllib  # Standard in 3.11 and later
ModuleNotFoundError: No module named 'tomllib'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ec2-user/runner/_work/executorch/executorch/pytorch/executorch/build/extract_sources.py", line 20, in <module>
    import tomli as tomllib
ModuleNotFoundError: No module named 'tomli'

CMake Error at CMakeLists.txt:101 (message):
  executorch: source list generation failed
```

Reviewed By: huydhn, cccclai

Differential Revision: D48423480

fbshipit-source-id: d9d5953a37493f94b8b01bf4735fab9b727aed6c