Add node_modules to iTasks build

This belongs to https://gitlab.science.ru.nl/clean-and-itasks/iTasks-SDK/-/merge_requests/517.

Downloading a GitLab artifact in Jenkins is not very neat, I agree, but avoids having to maintain an npm installation on the build nodes. Something similar was done in the abc-interpreter build. If we eventually move builds to GitLab this can easily be improved.

The build fails because the artifacts cannot be downloaded yet because the iTasks MR has not been merged yet.