ci: Cache downloaded nw.js tarball

See: #668 (closed)

ubuntu-16.04 build without tarball cached: 42 minutes 58 seconds

ubuntu-16.04 build with tarball cached: 42 minutes 53 seconds

You can see that the tarball was from the cache in the second build as the log says 'Found nwjs-sdk-v0.24.4-linux-x64.tar.gz'.