Upload screenshots from failed CI tests to where we can see them

[pozorvlak]

Currently they're put in /tmp, and then thrown away when the container is torn down at the end of the build. There are a few options for uploading them to Amazon S3 - see comments on #924.

I'd be happy to host them with my own AWS account, but the high-bus-number approach would be to host them somewhere controlled by the whole GS maintainer team.