macOS 11.0 support, Library Foundation not found

**Environment**
Installed GRR from DEB, version `3.4.0.1` on macOS Big Sur developer beta 4.
```
sw_vers
ProductName: macOS
ProductVersion: 11.0
BuildVersion: 20A5343i
```

**Describe the issue**
GRR fails to launch as a result of the missing Foundation (CTypes) library.

**Error logs**
``` bash
I0814 12:26:37.996510 4527623680 client_logging.py:116] Writing log file to /var/log/GRRlog.txt
INFO:2020-08-14 12:26:37,996 client_logging:116] Writing log file to /var/log/GRRlog.txt
Traceback (most recent call last):
File "site-packages/grr_response_client/client.py", line 102, in
File "site-packages/absl/app.py", line 299, in run
File "site-packages/absl/app.py", line 250, in _run_main
File "site-packages/grr_response_client/client.py", line 92, in main
File "site-packages/grr_response_client/comms.py", line 977, in __init__
File "site-packages/grr_response_client/comms.py", line 142, in __init__
File "site-packages/grr_response_client/comms.py", line 175, in _GetProxies
File "site-packages/grr_response_client/client_utils_osx.py", line 36, in FindProxies
File "site-packages/grr_response_client/osx/objc.py", line 202, in __init__
File "site-packages/grr_response_client/osx/objc.py", line 133, in __init__
File "site-packages/grr_response_client/osx/objc.py", line 95, in LoadLibrary
File "site-packages/grr_response_client/osx/objc.py", line 73, in SetCTypesForLibrary
grr_response_client.osx.objc.ErrorLibNotFound: Library Foundation not found
```

**Additional context**
Manual installation of Xcode 12 beta Command Line Developer Tools resolved many similar issues for Ruby-based projects, but not Python.