builder: internals: fix incomplete chown walk when fixing permissions

This patch fixes the permission fixing code used by `addContext`, which
would not be responsible for `Lchown`-ing top-level directories added to a
destination that didn't exist prior to untar-ing the context.

Closes #9608

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)