ci: Cache downloaded nw.js tarball

We can persist files between builds in the /cache folder, provided it's enabled in the .gitlab-ci.yml config. (Details at https://docs.gitlab.com/ee/ci/caching/)

For the CI script to do this, it needs to know the nw.js filename for the current platform. This info is currently determined in the tar_em_up.sh script. I propose that we move that code into a separate nwjs_version_for_platform shell script which we could then call both from tar_em_up.sh and from .gitlab-ci.yml.