[Refactor] Move the Linux CI jobs to OSDC runners

[huydhn]

Stack from ghstack (oldest at bottom):

• [Refactor] Bound GPU test parallelism and reap strays #4529
• [Refactor] Fix the unittest setup scripts for OSDC containers #4528
• -> [Refactor] Move the Linux CI jobs to OSDC runners #4527

Moves the Linux test and lint jobs across 11 workflows onto OSDC pods.

before	after
linux.g5.4xlarge.nvidia.gpu	mt-l-x86aavx2-11-41-a10g
linux.g6.4xlarge.experimental.nvidia.gpu	mt-l-x86aavx2-11-41-l4
linux.12xlarge	mt-l-x86iavx512-48-384
linux.4xlarge	mt-l-x86iavx512-16-128

Each pod sits on the same instance the label it replaces resolved to, so GPU
model and count are unchanged and only the Kubernetes overhead comes off.
linux_job_v2 -> v3 comes with it, per job: OSDC rejects a job with no
container.

The lint jobs set no runner: at all, so they were quietly taking v2's
linux.2xlarge default. They now pin an OSDC pod explicitly.

On OSDC the checkout runs inside the job container and no stock nvidia/cuda
image ships git, so 42 jobs move to the images test-infra publishes for
this -- the same bases plus git:

nvidia/cuda:12.4.0-devel-ubuntu22.04 -> osdc-cuda:cuda12.4.1-cudnn-devel-ubuntu22.04
nvidia/cuda:13.0.2-cudnn-devel-ubuntu24.04 -> osdc-cuda:cuda13.0.3-cudnn-devel-ubuntu24.04
nvidia/cuda:12.8.x-*-ubuntu22.04 -> osdc-cuda:cuda12.8.1-cudnn-devel-ubuntu24.04
nvidia/cuda:11.8.0-cudnn8-devel-ubuntu22.04 -> osdc-cuda:cuda11.8.0-cudnn8-devel-ubuntu22.04
pytorch/pytorch:2.8.0-cuda12.9-cudnn9-devel -> osdc-cuda:cuda12.9.2-cudnn-devel-ubuntu24.04

unittests-robohive keeps nvidia/cudagl:11.4.0-base -- it is the only image
providing the EGL stack that suite renders with -- so it runs standalone
with an explicit git install instead of through v3.

test-linux-libs.yml's unittests-isaaclab stays on EC2: it calls a fork's
reusable workflow that cannot be confirmed to set a container.

docs.yml is handled in #4524. Wheel builds are in #4487.

Authored with Claude Code.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/4527

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[huydhn]

(Claude, on huydhn's behalf.) Superseded by the gh stack version of the same split: #4530 / #4531 / #4532. Same three commits, same content — this one was built with ghstack by mistake.