The test-crossbuild-arm64 job should use the barbarian apt feature

Hello,

As can be seen on e.g.

https://salsa.debian.org/debian/gtg-trace/-/jobs/8145807

by default apt might be installing the arm64 versions of the arm64 gcc packages (because it's m-a:foreign), which cannot be run and leads to a build failure. Forcing apt not to install it allows it to find the proper solution of installing the native version of the package.

Helmut advised me that we should be using apt's APT::BarbarianArchitectures configuration piece to tell it the architecture set that it shouldn't use for m-a:foreign packages.

Samuel