lib/Sbuild/Build.pm: don't copy in /usr/lib/apt/solvers/apt from the outside

The executable /usr/lib/apt/solvers/apt from the outside can come from a very different distro than the one in the chroot. Required library versions might just not exist. With a different glibc, one easily gets:

/usr/lib/apt/solvers/apt: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.32' not found (required by /usr/lib/apt/solvers/apt)

Instead, install apt-utils into the chroot if we are cross building to make a working version of /usr/lib/apt/solvers/apt available.

In the future, this problem can be solved by calling apt from the outside but such a feature is incompatible with the schroot backend.

Closes: #1088836