OvmfPkg: CI: use Python version from defaults template

OvmfPkg: CI: use Python version from defaults template

Use the default Python version from the defaults template
(.azurepipelines/templates/defaults.yml) in the Windows and
Linux CI jobs.

Previous changes to the CI job templates make it necessary
to specify a version number, if Python shall be pulled
at CI runtime.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chris Fernald <chfernal@microsoft.com>