Almost 100% of dependencies are hosted on npm. So the CI should fail and you wait for npm to fix the issue. In the unlikely scenario that npm it gone forever,

Almost 100% of dependencies are hosted on npm. So the CI should fail and you wait for npm to fix the issue. In the unlikely scenario that npm it gone forever, you can still get the dependencies from a previous build or a random developer laptop, or Github. If you must build and deploy a version of your software while npm is down, which is unlikely but may happen, well you may have to skip the CI and build from a developer laptop.