CI: Stop building QEMU in general

CI: Stop building QEMU in general

We spend an awful lot of CI time building QEMU, even though most changes don't
touch the subset of tools/libs/ used by QEMU.  Some numbers taken at a time
when CI was otherwise quiet:

                       With     Without
  Alpine:              13m38s   6m04s
  Debian 12:           10m05s   8m10s
  OpenSUSE Tumbleweed: 11m40s   7m54s
  Ubuntu 24.04:        14m56s   8m06s

which is a >50% improvement in wallclock time in some cases.

The only build we have that needs QEMU is alpine-3.18-gcc-debug.  This is the
build deployed and used by the QubesOS ADL-* and Zen3p-* jobs.

Xilinx-x86_64 deploys it too, but is PVH-only and doesn't use QEMU.

QEMU is also built by CirrusCI for FreeBSD (fully Clang/LLVM toolchain).

This should help quite a lot with Gitlab CI capacity.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>