Add non-default option to `./depends` build system to refuse non-z.cash downloads; then reconfigure CI.

[nathan-at-least]

Let's add a new non-default option to the ./depends system that fails any time it would otherwise fetch dependencies from upstream, eg make -C ./depends NO_FETCH_UPSTREAM=1. Users won't get this behavior by default, so that if our website is down they can still build.

Meanwhile, we should enable this option in the buildbot tests, especially for merging onto master. This will ensure buildbot fails until we've correctly hosted all dependencies on our website.