ci : add missing Linux label to cpu-x64-high-perf runner (#23958)

Fixes: https://github.com/ggml-org/llama.cpp/pull/23927#discussion_r3332213086

The cpu-x64-high-perf job was missing the Linux label in its runs-on
specification, causing the runner to not be discovered. All other
self-hosted Linux jobs include this label.

Assisted-by: llama.cpp:local pi