ci: onboard GB200 testing

[ko3n1g]

Summary

Adds GB200 (ARM64/GCP) support to the container build and test pipeline.

Container build — matrix pattern (same as Megatron-Bridge / Megatron-LM):

• Adds `container-registry-gb200` env var pointing to the GAR repository for ARM64 images
• Adds `cicd-compute-build-matrix` job that outputs a two-entry matrix: AMD64 → Azure ACR on `{runner_prefix}-gpu-x2`, ARM64 → GAR on `nemo-ci-gcp-gpu-x2`
• Refactors `cicd-container-build` to use the matrix strategy — inlines steps previously delegated to `.github/actions/build-container`; Azure login steps are gated on `contains(matrix.registry, 'azure')` so the GCP runner never touches ACR

Test registration and discovery:
Extends Automodel's existing static-matrix approach with a parallel `cicd-e2e-tests-gb200` job. Same shape as `cicd-e2e-tests` (explicit `test-name` / `test-folder` / `timeout` per entry), fixed to `nemo-ci-gcp-gpu-x2` and the GAR image. Engineers add or remove GB200 test entries directly in the matrix — no scripts, no scanning.

Bug fix:
`Azure ACR Login` in `test-template/action.yml` was unconditional — added `if: inputs.has-azure-credentials == 'true'` guard to prevent `az` failures on GCP runners.

Build matrix

Entry	Runner	Registry	Platform
aws	`{runner_prefix}-gpu-x2`	`nemoci.azurecr.io`	linux/amd64
gcp	`nemo-ci-gcp-gpu-x2`	GAR (`us-east4-docker.pkg.dev/…/automodel`)	linux/arm64

Prerequisite: NeMo-CI-TF-States#72 must be merged first to provision the `automodel` GAR repository.

Test plan

• Trigger `workflow_dispatch` — confirm AMD64 build pushes to Azure ACR and ARM64 build pushes to GAR on the GCP runner
• `cicd-e2e-tests-gb200` jobs appear and run on `nemo-ci-gcp-gpu-x2` pulling the GAR image
• H100 test jobs unaffected (still use `nemoci.azurecr.io`)

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