fix(roce_backend): build MOFED ISO filename from target architecture

[100milliongold]

Problem

roles/roce_backend/vars/main.yml hardcodes the ISO filename, including the
distro version and architecture that the two variables above it already carry:

mofed_version: "24.10-4.1.4.0"
mofed_file_name: "MLNX_OFED_LINUX-24.10-4.1.4.0-ubuntu24.04-x86_64.iso"

The ISO is published per distro release and per architecture, so this name is
wrong on aarch64, and also wrong on Ubuntu 22.04 even on x86_64.

Both are in scope. README.md:43 lists "Ubuntu 22.04 LTS and 24.04 LTS", and
line 52 names them again "for generic Kubernetes and Slurm deployments";
docs/deepops/testing.md:126 says "DeepOps currently uses Ubuntu 22.04 and
Ubuntu 24.04 for setup and Molecule GitHub Actions"; and per-release vars files
exist for both (roles/nvidia-dgx/vars/ubuntu-22.04.yml,
roles/nhc/vars/ubuntu-22.04.yml).

Change

Build the name from mofed_version, ansible_distribution_version, and
ansible_architecture — the same expression roles/mofed already uses
(roles/mofed/defaults/main.yml), so the two roles stop disagreeing.

No architecture alias map is introduced here, unlike PR ① and ②. The point of
this change is to stop disagreeing with roles/mofed, and that role
(roles/mofed/defaults/main.yml:6) also interpolates ansible_architecture
directly without normalising aliases — adding a map here would reintroduce the
mismatch. The difference has no practical effect: ansible_architecture is
uname -m, which never reports arm64 on Linux. For the record, an arm64
input would render ...-ubuntu24.04-arm64.iso, which returns HTTP 404; only the
aarch64 name exists.

README updated to match, since it quotes this block verbatim.

Verification

All four combinations of {24.04, 22.04} × {x86_64, aarch64} are published, so
every name this expression renders resolves to a real ISO:

ubuntu24.04 x86_64   HTTP 200  content-length: 324743168   (today's hardcoded name)
ubuntu24.04 aarch64  HTTP 200  content-length: 318736384
ubuntu22.04 x86_64   HTTP 200  content-length: 328933376
ubuntu22.04 aarch64  HTTP 200  content-length: 322516992

The aarch64 24.04 ISO reports content-type: application/x-iso9660-image. The
three names other than the first are unreachable with the hardcoded value.