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

EmulatorPkg/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, an EmulatorPkg linking step fails like this:

> INFO - "gcc" -o
> /home/vsts/work/1/s/Build/EmulatorIA32/DEBUG_GCC5/IA32/Host -m32
> -L/usr/X11R6/lib
> -Wl,--start-group,@/home/vsts/work/1/s/Build/EmulatorIA32/DEBUG_GCC5/IA32/EmulatorPkg/Unix/Host/Host/OUTPUT/static_library_files.lst,--end-group
> -lpthread -ldl -lXext -lX11
> INFO - /usr/bin/ld: skipping incompatible
> /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a when searching for -lgcc
> INFO - /usr/bin/ld: cannot find -lgcc
> INFO - /usr/bin/ld: skipping incompatible
> /usr/lib/gcc/x86_64-linux-gnu/9/libgcc.a when searching for -lgcc
> INFO - /usr/bin/ld: cannot find -lgcc
> INFO - collect2: error: ld returned 1 exit status
> INFO - make: *** [GNUmakefile:421:
> /home/vsts/work/1/s/Build/EmulatorIA32/DEBUG_GCC5/IA32/EmulatorPkg/Unix/Host/Host/DEBUG/Host]
> Error 1

So for now, stick with the previous Ubuntu environment, which continues to
be supported, per
<https://github.com/actions/virtual-environments/issues/1816>.

The following ticket has been opened about this particular issue:
<https://github.com/actions/virtual-environments/issues/2324>.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Ray Ni <ray.ni@intel.com>
Message-Id: <20201221031930.1799-1-bob.c.feng@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: update the commit message to refer to GCC rather than
 to QEMU]