iso: pin NVIDIA driver/DCGM to DGX OS 7.5.0-validated combo

590.48.01/DCGM 4.6.0-1 was never a deliberate compatibility choice — the
driver version traces back to a CDN 404 on the originally pinned
550.54.15 (commit 559fc29), and DCGM was bumped twice since purely
because NVIDIA pulled the previously-pinned core package from the repo
(6dee8f3, b2e177a). The 590 branch only has two point releases so far,
and its nvvs plugin fails dcgmi diag with "Detected driver major
version 0 is not between the required versions 346 and 2000" on H100 —
confirmed via a staging support bundle.

Switch to 580.126.20 / DCGM 4.5.2-1, the combo NVIDIA's own DGX OS 7.5.0
release notes validate for H100. Both packages confirmed present in the
NVIDIA CUDA debian12/x86_64 apt repo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

iso/README.md

@@ -48,7 +48,7 @@ sh iso/builder/build-in-container.sh --cache-dir /path/to/cache
 - The builder image is automatically rebuilt if the local tag exists for the wrong architecture.
 - The live ISO boots with Debian `live-boot` `toram`, so the read-only medium is copied into RAM during boot and the runtime no longer depends on the original USB/BMC virtual media staying present.
 - Target systems need enough RAM for the full compressed live medium plus normal runtime overhead, or boot may fail before reaching the TUI.
-- The NVIDIA variant installs DCGM 4 packages matched to the CUDA user-mode driver major version. For driver branch `590` / CUDA `13.x`, the package family is `datacenter-gpu-manager-4-cuda13` rather than legacy `datacenter-gpu-manager`.
+- The NVIDIA variant installs DCGM 4 packages matched to the CUDA user-mode driver major version. For driver branch `580` / CUDA `13.x`, the package family is `datacenter-gpu-manager-4-cuda13` rather than legacy `datacenter-gpu-manager`.
 - Override the container platform only if you know why:
 
 ```sh