don't let anything download files into the repo

[onlyjsmith]

I think this line, and maybe others, downloads external files into the folder where the code is.

This is bad!

We need to make use of the tempdir and tempfile commands to write to locations outside the code repo.

There's a closely-related issue about caching, which I'll open separate issue on.

[onlyjsmith]

https://rdrr.io/r/base/tempfile.html is a place to start

[onlyjsmith]

See also disarm-platform/algorithms-issues#1

[HughSt]

Related to #5