ci: debian: Disable Debian bullseye ppc64el and s390x

Debian bullseye cannot install dependencies for ppc64el and s390x
without pulling libc6-dev from that arch, which conflicts with base/host
libc6, thus just disable them for now:

  apt install -y --no-install-recommends autoconf:s390x autoconf-archive automake:s390x
    libdbus-1-dev:s390x libglib2.0-dev:s390x libnl-3-dev:s390x libnl-genl-3-dev:s390x libtool:s390x gcc-s390x-linux-gnu
  ...
  libgcc-s1:s390x : Depends: libc6:s390x (>= 2.2.4) but it is not installable
  libnl-3-200:s390x : Depends: libc6:s390x (>= 2.8) but it is not installable
  libnl-genl-3-200:s390x : Depends: libc6:s390x (>= 2.4) but it is not installable

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>