https://salsa.debian.org/apt-team/apt/-/blob/main/apt-pkg/contrib/fileutl.cc#L398

it would be good to make this configurabel with something similar to
`FileMode`.

We are using `apt-ftpachive` in a shared ennvironment. We are using
`umask 0002` to give group-membery rwx-access by default.
Our top-level directory has mode 02775 and group "build", so that every
path created below it is again owned by that group and has the SGID-bit
set.

But directories and files created by `apt-ftparchive` lack
write-permission for the group:
- Permissions for files can be configured via `FileMode`, which defaults
  to 0644.
- But there is nothing for directories, which is hard-coded to 0755.