Remove usage of environment variables in wheel building (#9583)

### Summary

We seem to be using a combination of CMAKE_ARGS and environment
variables when creating wheels. Ultimately, CMake only uses the cmake
args, however we redefine some of these flags as env vars to help
`setup.py` determine if a certain feature is turned on. Specifically, it
looks for pybinding vars to bundle pybindings.

Let's remove this redundancy and just use the CMAKE_ARGS as the single
source of truth. For more details and other considerations, see
https://github.com/pytorch/executorch/pull/9494 (abandoned).

Note that even in the wheel building jobs, we use cmake args instead of
environment variables to control features:


https://github.com/pytorch/executorch/blob/644b7ddf14180d97e348faa627f576e13d367d69/.ci/scripts/wheel/envvar_base.sh#L20


https://github.com/pytorch/executorch/blob/644b7ddf14180d97e348faa627f576e13d367d69/.ci/scripts/wheel/envvar_macos.sh#L14-L15

### Test plan

build + check CMakeCache.txt to ensure flags are set

```bash
# Expected: EXECUTORCH_BUILD_PYBIND=OFF EXECUTORCH_BUILD_XNNPACK=OFF EXECUTORCH_BUILD_COREML=OFF
$ rm -rf pip-out dist && ./install_executorch.sh --pybind off

# Expected: EXECUTORCH_BUILD_PYBIND=ON EXECUTORCH_BUILD_XNNPACK=ON EXECUTORCH_BUILD_COREML=OFF
$ rm -rf pip-out dist && ./install_executorch.sh

# Expected: EXECUTORCH_BUILD_PYBIND=ON EXECUTORCH_BUILD_XNNPACK=OFF EXECUTORCH_BUILD_COREML=ON
$ rm -rf pip-out dist && ./install_executorch.sh --pybind coreml

# Expected: EXECUTORCH_BUILD_PYBIND=ON EXECUTORCH_BUILD_XNNPACK=ON EXECUTORCH_BUILD_COREML=ON
$ rm -rf pip-out dist && ./install_executorch.sh --pybind xnnpack coreml

# Throws an error
$ rm -rf pip-out dist && ./install_executorch.sh --pybind coreml off
```


cc @larryliu0820 @lucylq