ci: bump Linux CUDA slim container to gha-convention base runner

[ndizazzo]

Summary

Pins the Linux CUDA slim matrix entry's container: image to the new digest published by Mesh-LLM/mesh-llm-runner-images #8 — the refactor to FROM ghcr.io/actions/actions-runner base + GHA convention /__e symlinks.

Before	After
sha256:c5b85ef5...	sha256:295341c6c9f17c9eb69281fd454bda953799406d6915f472c914fb5f024a88ed

Why

The old image was built from mcr.microsoft.com/dotnet/runtime-deps:8.0-noble and was missing the GHA convention mount point /__e/node24/bin/node. Once the ARC runner pod has a working docker CLI (baked by the new base image) and the host-path docker socket from carrack mounted at /var/run/docker.sock, the actions/runner agent execs marketplace JS actions via:

docker exec <container> /__e/node24/bin/node <runner-temp>/actions/checkout/dist/index.js

The old image lacks /__e/node24/bin/node, so this fails with:

OCI runtime exec failed: exec failed: unable to start container process: exec: "/__e/node24/bin/node": stat /__e/node24/bin/node: no such file or directory
``\]

— confirmed at `Linux CUDA slim` step 3 (`actions/checkout@v5`) on run 30146423269 job 89658307286 (in stanton-patio51 cluster build environment).

Reference: [actions/runner-container-hooks#347](https://github.com/actions/runner-container-hooks/issues/347) — exact same failure mode on K8s self-hosted DinD.

The new image inherits `ghcr.io/actions/actions-runner:latest` (Ubuntu 24.04 with built-in `runner` user, `/home/runner/externals/{node20,node24}/bin/node`, `/usr/bin/docker` CLI, and `/__e/node24` symlinks) and layers CUDA 12.9.2 + Rust + just + sccache + pnpm on top.

## Pairs with

- **stanton-patio51** commit [`31a661b`](https://github.com/ndizazzo/crusader-patio51/commit/31a661b) — bumps the corresponding self-hosted ARC runner pod image to `sha256:2dd446840d607f893ab1a747668a1554ba5cece64ec5a6dbbaa725bf3479c2f3` (self-hosted cuda12 amd64), drops the now-redundant docker-cli-installer initContainer, keeps the supplementalGroups + host-path docker socket mount.

## Verification plan

- This PR's follow-up push triggers PR Builds (this workflow).
- Expect: `Linux CUDA slim` job runs to completion (no longer errors at `actions/checkout@v5`) on stanton-patio51 self-hosted AMD64 scale-set runner.
- Other matrix cells unchanged (ROCm slim, Vulkan, macOS, Linux CPU) — they use `runs_on: [ubuntu-24.04]` (GitHub-hosted) and are not affected by this digest swap.


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

## Summary by CodeRabbit

* **Chores**
  * Updated the CUDA build environment to use a refreshed, verified container image.
  * Improved reliability and consistency for Linux CUDA build checks.

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

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 4160455f-5ee4-4573-b3e1-5fc43ca1fdad

📥 Commits

Reviewing files that changed from the base of the PR and between cdd793b and a26e6b8.

📒 Files selected for processing (1)

• .github/workflows/pr_builds.yml

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📝 Walkthrough

Walkthrough

The CUDA slim Linux build matrix entry now references an updated pinned digest for the mesh-llm-cuda-runner container image.

Changes

CUDA runner update

Layer / File(s)	Summary
Update CUDA runner image pin .github/workflows/pr_builds.yml	The CUDA slim matrix entry uses a new pinned container image digest.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: i386, michaelneale

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the CI change to the Linux CUDA slim container and its new base runner image.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/cuda-slim-digest-bump-gha-convention

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