This is the symptom:

Error: Unable to download artifact(s): Unable to download and extract artifact: Artifact download failed after 5 retries.

According to Copilot:

Self‑hosted runners (especially ARM) are the #1 cause
The GitHub issue report shows this exact failure happening on Namespace self‑hosted ARM64 runners. The download step redirects to an Azure Blob Storage URL and then fails repeatedly.

uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8

That SHA corresponds to v8 of the artifact actions, which is the new implementation built on the rewritten artifact toolkit.
And unfortunately: v8 is currently the most failure‑prone release, especially on self‑hosted runners, ARM64, and any environment with imperfect outbound networking.
This is not speculation — it matches the exact failure mode you’re seeing:

redirect to Azure blob URL
5 retry attempts
extraction never starts
“Unable to download artifact(s)”

This is the same regression that caused widespread breakage when v5 of the toolkit shipped.
The fix that works for almost everyone
Pin to v4 instead of v8