ctrl+c sends SIGINT to bubblwrap instead of child process

Here is a dumb python program where the issue happens:

```
import signal
import asyncio

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)

def interrupt():
s = input("test")
print(s)
loop.stop()

loop.add_signal_handler(signal.SIGINT, interrupt)

loop.run_forever()

loop.remove_signal_handler(signal.SIGINT)

loop.close()
```

On host, running this program, then pressing ctrl+c, a prompt shows up, then I can type some text and enter. The text is printed again and the program exits.

Now, if I run through `bwrap --ro-bind / / -- /usr/bin/python3 test.py`, I get the error:
```
handle:
Traceback (most recent call last):
File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "test-interrupt.py", line 8, in interrupt
s = input("test")
EOFError
```

This is because is bwrap is part of foreground process group. Instead it should be the child process as to be the foreground process group.