Changes

testenv: qemu: ignore builtin modules

In debian 13, the virtio_pci kernel module is now builtin. Adjust the
code paths that add kernel modules to the initramfs and load them to
ignore builtin modules.

This fixes the ttcn3-ggsn-test-kernel-latest job job, which currently
fails with:

  + modprobe -a --dry-run --show-depends --set-version=6.12.41+deb13-amd64 virtio_net virtio_pci
  + sort -u
  + cut -d   -f 2
  + local files=virtio_pci
  /lib/modules/6.12.41+deb13-amd64/kernel/drivers/net/net_failover.ko.xz
  /lib/modules/6.12.41+deb13-amd64/kernel/drivers/net/virtio_net.ko.xz
  /lib/modules/6.12.41+deb13-amd64/kernel/net/core/failover.ko.xz
  + qemu_initrd_add_file virtio_pci /lib/modules/6.12.41+deb13-amd64/kernel/drivers/net/net_failover.ko.xz /lib/modules/6.12.41+deb13-amd64/kernel/drivers/net/virtio_net.ko.xz /lib/modules/6.12.41+deb13-amd64/kernel/net/core/failover.ko.xz /usr/lib/modules/6.12.41+deb13-amd64/modules.alias
  + local i
  + local cp=cp --no-dereference --recursive --parents
  + cp --no-dereference --recursive --parents virtio_pci <https://jenkins.osmocom.org/jenkins/job/ttcn3-ggsn-test-kernel-latest/1665/artifact/logs/osmo_ggsn_v4_only/ggsn/_initrd>
  cp: cannot stat 'virtio_pci': No such file or directory

Change-Id: Ied8acf18d0a1f47fac805619d15896918a365ff2

testenv: fix TESTENV_INSTALL_DIR for --binary-repo

When --binary-repo is set, "/" must be used as install directory,
without any suffix for the distribution.

Fix for:
  realpath: /-debian-trixie/usr/lib/osmo-s1gw: No such file or directory

Fixes: 2152d512 ("testenv: use separate cache dirs per distro")
Change-Id: I1e1ca65fd6a7d77c751dd7138015ff474f8ae576