[RFC] Fix setting up prebuilt dependencies.

Test build is at https://travis-ci.org/fwalch/bot-ci/builds/42436559, still running, but looks like it works.
Hm. Here, `_call_function` is indeed not used. I only did it like this because using `_call_function` wouldn't work: there are two parameters (`_setup_deps` and `"${deps_path}"`). I'll have a look if using `source` would make things easier, but I'd leave that for a separate PR. Also, not using `_call_function` means that using dashes (`nightly-x64`) won't work anymore, and this will require PRs to repos using the nightly build.