tests/uefi-test-tools/build.sh: work around TianoCore#1607

tests/uefi-test-tools/build.sh: work around TianoCore#1607

The edk2-stabe201903 release introduced Python3 support to edk2's
BaseTools; however the Python3 enablement breaks in a corner case (which
is nevertheless supported by the edk2 community), namely the in-module
parallelization that we utilize.

This is tracked under
<https://bugzilla.tianocore.org/show_bug.cgi?id=1607>. For now, work
around the issue (in advance) by forcing Python2. (The workaround is a
no-op before we move to edk2-stabe201903 in the roms/edk2 submodule.)

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>