CI: point LLM container build at the new release wheel repo (#2043)

## What?

Point `nixl-ci-build-llm-container` at the new release wheel location:

- repo: `sw-dynamo-nixl-pypi-local` -> `sw-nbu-swx-nixl-pypi-local`
- path: drop the `/<arch>` segment (the release layout is flat, arch is
in the wheel filename)
- update the matching JJB descriptions

## Why?

Release wheels moved to
`sw-nbu-swx-nixl-pypi-local/release/<ver>/<sha8>/`, so every run 404s in
`Download Wheels` and blocks release verification.

Verified with test build
[#24](https://nbuprod.blsm.nvidia.com/nbu-swx-nixl-main/blue/organizations/jenkins/nixl-ci-build-llm-container/detail/nixl-ci-build-llm-container/24/pipeline/)
(`ci_refspec=refs/pull/2043/head`, vllm-cu13, 1.4.0/rc1/51a42f86):
wheels download on both arches, smoke test prints `vllm 0.22.1 nixl
1.4.0`, multi-arch manifest published. Test tag removed afterwards.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Chores**
* Updated build configuration to use the consolidated wheel repository
and release directory structure.
* Improved architecture-specific wheel discovery during container
builds.
* Updated build documentation to reflect the current artifact location
and release identification format.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->