ArmVirtPkg/PlatformCI: stick with "ubuntu-18.04" for now

ArmVirtPkg/PlatformCI: stick with "ubuntu-18.04" for now

"vm_image: 'ubuntu-latest'" now refers to Ubuntu Focal (20.04LTS), not
Ubuntu Bionic (18.04LTS), according to
<https://github.com/actions/virtual-environments/issues/1816>.

In Focal, the "qemu" package is a dummy package with no dependencies, and
so the actual emulators are not pulled in. Compare:

  https://packages.ubuntu.com/bionic/qemu
  https://packages.ubuntu.com/focal/qemu

This causes CI runs to fail.

It would be best to switch to the "qemu-system" package name, which
continues to depend on the emulators:

  https://packages.ubuntu.com/bionic/qemu-system
  https://packages.ubuntu.com/focal/qemu-system

However, while that package does make the emulators available, the
emulators crash. So for now, stick with the previous Ubuntu environment,
which continues to be supported, per
<https://github.com/actions/virtual-environments/issues/1816>.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201217204049.26817-2-lersek@redhat.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>