add MEDIASOUP_MAX_CORES to limit NUM_CORES during build

[mattkrick]

Thanks so much for mediasoup. It's absolutely the best & your dedication to the community is amazing. OSS is hard & thankless, so thank you.

When building in a virtual CI environment, nproc could return a very large number of cores. This may cause the kernel to run out of memory when building. While #448 may be a more desirable longterm solution, I understand the additional maintenance cost isn't trivial.

This PR allows folks to set a max number of cores to use.
It works like this: MEDIASOUP_MAX_CORES=8 yarn.
I namespaced the variable because it's possible that other packages might do similar things.

This is hard to test, so I'm including 2 circleCI builds as proof (see "Install Dependencies"):

• before: https://app.circleci.com/pipelines/github/ParabolInc/parabol/724/workflows/9b919a23-f992-419c-a273-ea467290f3ad/jobs/9604
• after: https://app.circleci.com/pipelines/github/ParabolInc/parabol/735/workflows/a6fd256e-a967-4f74-bd04-d13bc0d54672/jobs/9616

[ibc]

Thanks a lot.

[ibc]

Released in 3.6.20