EmulatorPkg: Use windows-2019 VM image

EmulatorPkg: Use windows-2019 VM image

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3890

The YAML file previously specified "windows-latest" which recently
moved to VS2022. To continue using VS2019 (at least in the short
term), the VM image needs to be explicitly set to "windows-2019".

Cc: Andrew Fish <afish@apple.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Nickle Wang <nickle.wang@hpe.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>