checkbox backend doesn't run when run from source

This use case is not that frequent but it's pretty annoying when it hits.

If you try to run checkbox from source, the backend can't be found by policykit, so it doesn't run. There are three consequences:

- Error indicating "./backend not found"
- Checkbox attempts to spawn the backend three time swith a 30-second timeout, which is annoying when trying to quickly test something.
- Jobs with user: will NOT run.

How to reproduce:

bzr branch lp:checkbox
cd checkbox/checkbox-old
bin/checkbox-qt -w data/whitelists/default.whitelist

A workaround (which also gives a hint as to the solution) is to specify the backend command's full path so pkexec is happy finding it:

bin/checkbox-qt --config=checkbox/plugins/backend_info/command=$(pwd)/backend