Use dh_fixperms for Debian package permissions

Description

The .deb package currently contains directories with 777 permissions (/, /usr/, /usr/share/, /usr/bin/). While this doesn't cause issues in practice (these directories already exist on target systems), it's not best practice for Debian packaging.

This is a follow-up to the comment: https://gitlab.com/gitlab-org/gitlab-runner/-/issues/39026#note_3045351863.

Actually, the permissions of /usr/share/gitlab-runner are fixed - BUT:

➜ /tmp dpkg-deb -c gitlab-runner_18.8.0-1_amd64.deb | grep drwxrwxrwx >drwxrwxrwx 0/0 0 2026-01-15 17:32 ./ drwxrwxrwx 0/0 0 2026-01-15 17:32 ./usr/ drwxrwxrwx 0/0 0 2026-01-15 17:32 ./usr/share/ drwxrwxrwx 0/0 0 2026-01-15 17:32 ./usr/bin/ ➜ /tmp

this is just not an issue as the directories exist already - the gitlab-runner subdirectory had the same permissions before. Please use dh_fixperms when you are building debian packages.

Proposed solution

Use dh_fixperms equivalent when building packages, or ensure packaging/root/ structure is created with correct permissions from the start.