autopkgtest unshare-qemuwrapper doesn't build chroot with sbuild from lunar-proposed

The autopkgtest `d/t/unshare` uses `sbuild`, which fails on lunar
(sbuild 0.83.1ubuntu1) as the chroot setup cannot find `adduser`
(not in the chroot as it's now an alternative dependency in `apt`).

This is fixed in lunar-proposed (sbuild 0.84.1ubuntu2; commit [1])
since it no longer depends on `adduser` in the chroot.

[1] https://salsa.debian.org/debian/sbuild/-/commit/0d35fa019a5c6d51e0928987a87849766b0f35e6

However, `d/t/unshare-qemuwrapper` runs `d/t/unshare` in a qemu vm,
but its image still does _not_ ship `sbuild` from lunar-proposed,
thus `unshare` fails anyway (and its parent `unshare-qemuwrapper`).

The reason is, even though the apt _sources_ are used to build the
vm disk image thus `lunar-proposed` is _available_, it is not used
because apt _preferences_ (`autopkgtest --apt-pocket=proposed=src:...`)
are not copied in.

This is now required as lunar has -proposed pocket w/ NotAutomatic.

Just checking for/copying the apt preferences file/dir address it:
+ mmdebstrap ... --setup-hook=sync-in /etc/apt/preferences.d/ ...

Tests:
=====

$ autopkgtest-buildvm-ubuntu-cloud -a amd64 -r lunar

Before:

$ autopkgtest sbuild \
  -B -U --apt-pocket=proposed=src:sbuild -- qemu autopkgtest-lunar-amd64.img
...
autopkgtest [15:24:09]: test unshare-qemuwrapper: [-----------------------
...
+ mmdebstrap --variant=apt --mode=root --verbose --setup-hook=/tmp/autopkgtest.NnTISR/autopkgtest_tmp/setup.sh --include=...
...
Setting up sbuild (0.83.1ubuntu1) ...
...

After:

$ autopkgtest ./sbuild_0.84.1ubuntu3.dsc \
  -B -U --apt-pocket=proposed=src:sbuild -- qemu autopkgtest-lunar-amd64.img
...
autopkgtest [15:39:50]: test unshare-qemuwrapper: [-----------------------
...
+ [ -e /etc/apt/preferences.d/autopkgtest-lunar-proposed ]
+ cat /etc/apt/preferences.d/autopkgtest-lunar-proposed
Package: *
Pin: release a=lunar
Pin-Priority: 990

Package: buildd libsbuild-perl sbuild sbuild-debian-developer-setup sbuild-qemu
Pin: release a=lunar-proposed
Pin-Priority: 995

Package: *
Pin: release a=lunar-updates
Pin-Priority: 990
...
+ mmdebstrap --variant=apt --mode=root --verbose --setup-hook=/tmp/autopkgtest.4gz0dQ/autopkgtest_tmp/setup.sh --setup-hook=sync-in /etc/apt/preferences.d/ /etc/apt/preferences.d/ --include=...
...
Setting up sbuild (0.84.1ubuntu2) ...
...