torbrowser: explicitly set paths to torrc-defaults and tor

This patch fixes an issue introduced by b897f825942b63ab790cad9ed66c8a79cbb3fc2d
where the bundled tor fails to run.

The error message claims that it cannot communicate with tor, but what actually
happens is that tor never runs at all, because it is invoked with a non-existent
rc file.  Specifying an absolute path to the torrc-defaults file fixes the
problem.  For good measure, we specify an absolute path to the tor executable
itself as well; we want as little autodetection logic as possible.

Note that users of torbrowser *must* remove the existing `~/.torbrowser4` folder
for this to take effect, otherwise torbrowser will continue to use extension
data from the previous release (this is why some existing users were able to
successfully use the new torbrowser version, see
https://github.com/NixOS/nixpkgs/pull/15854).