get dpkg lock in build-dep if cache was invalid again

get dpkg lock in build-dep if cache was invalid again

Regression introduced in a249b3e6fd798935a02b769149c9791a6fa6ef16, which
in the case of an invalid cache would build the first part unlocked and
later pick up the (still unlocked) cache for further processing, so the
system got never locked and apt would end up complaining about being
unable to release the lock at shutdown.

The far more common case of having a valid cache worked as expected and
hence covered up the problem – especially as tests who would have
noticed it are simulations only, which do not lock.

Closes: 814139
Reported-By: Balint Reczey <balint@balintreczey.hu>
Reported-By: Helmut Grohne <helmut@subdivi.de> on IRC