ci : add PYTEST_WORKERS=1 to fix server-self-hosted job

[danbev]

Overview

This commit adds the PYTEST_WORKERS=1 environment variable to the hf-jobs-t4-small:cuda13 runner steps.

Additional information

This is an attempt to address CI failure of this job that I might have introduced in Commit 42f0225 ("server : use pytest-xdist for server tests (#28298)").

Refs: https://github.com/ggml-org/llama.cpp/actions/runs/34126971262/job/101757819134

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: No

[CISC]

Ah, I was wondering what that failure was about...

[CISC]

I think metal job needs it too?

Edit: or strangely enough, at least it fails identically sometimes:
https://github.com/ggml-org/llama.cpp/actions/runs/34200062554/job/101976568181

[danbev]

I think metal job needs it too?

Ah yes it might, I'll take a look 👍

[danbev]

I tried to run this manually on my fork but it got queued so trying here:
https://github.com/ggml-org/llama.cpp/actions/runs/34215251161 ❌

New run which includes the metal-job updates as well:
https://github.com/ggml-org/llama.cpp/actions/runs/34217920814 ✔️

[ggerganov]

The matal job likely needs it as well: https://github.com/ggml-org/llama.cpp/actions/runs/34183828952/job/101928096964