GHA: adjust parallel job counts

GHA: adjust parallel job counts

Adjusts the `make -j` flag to match the latest GitHub-hosted runner
hardware specs[^1]:

 - `ubuntu-latest` on 4 CPU cores
 - `macos-latest` on 3 CPU cores

The processor count is ideally obtained from `nproc`, but setting env
vars from the current CI yaml files is not possible because they expect
literal strings.

[^1]: https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

Closes #12927