matrix-synapse: fix startup

Currently, `setuptools` isn't propagated automatically to python
packages[1] which causes the following error when starting
`matrix-synapse`:

```
Traceback (most recent call last):
   File "/nix/store/xxkds7821mrahfx75az0sq3ryf69m612-matrix-synapse-1.3.1/bin/.homeserver-wrapped", line 39, in <module>
     import synapse.config.logger
   File "/nix/store/xxkds7821mrahfx75az0sq3ryf69m612-matrix-synapse-1.3.1/lib/python3.7/site-packages/synapse/config/logger.py", line 27, in <module>
     from synapse.app import _base as appbase
   File "/nix/store/xxkds7821mrahfx75az0sq3ryf69m612-matrix-synapse-1.3.1/lib/python3.7/site-packages/synapse/app/__init__.py", line 18, in <module>
 E402
   File "/nix/store/xxkds7821mrahfx75az0sq3ryf69m612-matrix-synapse-1.3.1/lib/python3.7/site-packages/synapse/python_dependencies.py", line 19, in <module>
     from pkg_resources import (
 No module named 'pkg_resources'
```

[1] https://github.com/NixOS/nixpkgs/pull/68314