Fix permission issue on files in the debian directory

We have a mismatching build for Mullvad Browser 13.5 due to permissions on the files in the debian directory in the debian.tar.xz tarball: #41158 (comment 3041043)

This tarball is created by dpkg-buildpackage, which apparently keeps the permissions from the files. The files are created (from templates) by rbm, before starting the container, so they might have different permissions depending on the umask outside the container.