mautrix-telegram: mark as broken on darwin

Tests currently fail like this:

```
/nix/store/yslk7x7iw3hka6d33kmnba9sxaia4492-python3.7-mautrix-0.4.0/lib/python3.7/site-packages/mautrix/util/manhole.py:9: in <module>
    from socket import SOL_SOCKET, SO_PEERCRED
E   ImportError: cannot import name 'SO_PEERCRED' from 'socket' (/nix/store/81qani7sdir46gjwf3a3jr2cv1aggkz1-python3-3.7.5/lib/python3.7/socket.py)
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 1.73s ===============================
```

Those values don't seem to be available on the MacOS-version of that
module. As there's no workaround implemented in the source, I assume
that upstream doesn't intend to support darwin-alike platforms atm.

pkgs/servers/mautrix-telegram/default.nix

@@ -65,6 +65,7 @@ buildPythonPackage rec {
     homepage = https://github.com/tulir/mautrix-telegram;
     description = "A Matrix-Telegram hybrid puppeting/relaybot bridge";
     license = licenses.agpl3Plus;
+    platforms = platforms.linux;
     maintainers = with maintainers; [ nyanloutre ma27 ];
   };
 }