ts-xen-build-prep: Install newer NASM version, to build OVMF

ts-xen-build-prep: Install newer NASM version, to build OVMF

Recent versions of OVMF now need a version of NASM that is newer
than the one available on Debian oldstable/buster. They want to use
NASM 2.15.05 [1], which is available in Debian stable/bullseye. The
need to use a newer version started with d3febfd9ade3 ("MdePkg:
Replace Opcode with the corresponding instructions.").

There is no backport package available but the nasm package from
Debian Bullseye can easily be installed on Buster as it has few
dependencies and are already satisfied.

[1] https://github.com/tianocore/edk2/commit/6a890db161cd6d378bec3499a1e774db3f5a27a7
    ("BaseTools: Upgrade the version of NASM tool")

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>