logilab-packaging #3652139 rpm building is partially broken [validation pending]

it will manage to build one rpm (for one distribution), but fail for an other build. This is due to the fact do do chdir in a tempdir before calling mock, but we do not go back to the source directory, so the next rpm building step is attempted from a cwd that do not exists any more (in which no specfile can be found, for sure).