Make requirement for root explicit

According to debian/rules, part of the build must run as root (or fakeroot). This MR makes that requirement explicit such as a change in defaults for dpkg will not affect debian-installer.

This change explicitly requests the current status quo, so it has 0% change of regressing for the build as far as standard tooling goes. Even the version of dpkg in oldoldstable knows about the field, so all supported version of dpkg will handle it gracefully (older versions emit a warning, so even then they still cope "gracefully"). The only risk of regression I can think of would be if d-i had its own d/control parser that errored out on fields it did not know (I have not seen that yet in other packages, but then debian-installer is special in more than one way, so it would be the least surprising case to have a special case).

I would appreciate it if this was fixed in Trixie, since I am hoping we can make this change in Trixie. However, I appreciate that you might be in the middle of a d-i release and want to minimize the changes just to keep things stable. From that perspective, it can certainly wait a d-i release.

Related MBF: https://lists.debian.org/debian-devel/2024/11/msg00535.html