Infra: double the power of our Github Actions

[lrineau]

https://github.blog/2024-01-17-github-hosted-runners-double-the-power-for-open-source/

@SaillantNicolas Please check all of our Github Actions, using compilations with -j. Some of them have use -j $(getconf _NPROCESSORS_ONLN) or -j $(nproc), but a few of them hardcode -j2.

• cgal/.github/workflows/build_doc.yml
• cgal/.github/test.sh