Fix for CI macOS-latest-cmake-arm64

[HanClinto]

As suggested by @slaren ( #6576 (comment) ), disabling Metal for test invocation of eval-callback to fix CI build on macOS-latest-cmake-arm64

Example failure:
https://github.com/ggerganov/llama.cpp/actions/runs/8651988935/job/23723903645#step:5:4773

[HanClinto]

I would prefer it if I could add -ngl 0 only for the osx platform, but I'm not familiar enough with cmake to do that, and welcome suggestions / corrections.

[slaren]

I think -ngl 0 is fine, the github runners do not have a GPU anyway. And -ngl 0 is already the default for all backends except Metal. To test with a GPU it could be run in the ggml-ci (in ci/run.sh), but it's not important.

[phymbert]

Thanks, sorry I did not read the error log carefully in the PR as I was thinking the metal test randomly failed :/

[HanClinto]

Thanks, sorry I did not read the error log carefully in the PR as I was thinking the metal test randomly failed :/

Not a problem at all! The eval-callback program looks awesome, and I'm glad it's added to the suite -- nice job on that!