Fix iOS CI (#30327)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30327
### Summary
Seems like starting from macOS 10.15, we can no longer get access to the `Downloads` folder in our macOS machines.
```
permissionError: [Errno 1] Operation not permitted: '/Users/distiller/Downloads'
```
The fix is to change the conda download directory to ${HOME}
### Test Plan
- iOS jobs are back to normal
- Don't break other jobs
Test Plan: Imported from OSS
Differential Revision: D18717380
Pulled By: xta0
fbshipit-source-id: cad754076bf4ae5035741aa57a310ad87c76726e
