don't use FindFile for external Dir::Bin commands

We usually use absolute paths to specific the location of dpkg, apt-key
and the like, but there is nothing wrong with using just the command
name and instead let exec(3) make the lookup in PATH.

We had a wild mixture before, so opting for the more accepting option
out of the two seems about right especially as it makes no difference in
the default case as apt uses absolute paths.