Bump dist in `sources.list.d/tor.list` from `stable` to `bullseye`

Debian recently bumped the release branch stable from buster to bullseye, and with it comes some different available packages. Our Docker image is based on debian:stable-slim and gets the packages in Tor's deb repo from stable.

The problem is, Tor's deb repo hasn't updated the dependencies to install Tor to follow this bump from buster to bullseye, and leaving the Dockerfile as-is will cause the building process to fail as it fails in installing Tor. The problematic dependencies is libevent-2.1-6 as it is not available in bullseye. But the Tor repo for bullseye, instead of depending on libevent-2.1-6, correctly depends on the available libevent-2.1-7.

While we can wait for the deb repo's maintainer to correct the dependencies, we can just change the dist we pull from to bullseye instead of stable so we can build the image in the meantime.