ci: stabilize native ARM jobs

[zevorn]

Summary

• skip redundant Sphinx documentation generation in the native ARM build while retaining x86 documentation coverage
• limit only the ARM Meson check job to two concurrent tests

Failure evidence

• build job 87516181005: Sphinx multiprocessing worker exited with EOFError while generating man pages
• check job 87516181197: s390x TLS migration exceeded its 120-second status wait under concurrent qtest load
• the failing merge commit and the preceding successful run have identical source trees, confirming resource-sensitive CI failures rather than a QEMU source regression

Validation

• actionlint passes after excluding the workflow file's pre-existing SC2046 warnings
• matrix option assertions pass through YAML parsing
• scripts/checkpatch.pl reports 0 errors and 0 warnings for both commits