It turns out that I'd configured Jenkins to watch the squeak-ci repo _on any branch_. As a result, my experimental will-ask-Dave-to-review-later branch ended up prematurely running on Jenkins.
I've now reconfigured SqueakTrunk and SqueakTrunkOnInterpreter to watch only the master branch.
The experimental stuff is around downloading (if necessary) a specified version of Cog rather than storing it in the repo. I've only worked on the Linux builtastic.sh script, but you can see it here: https://github.com/frankshearar/squeak-ci/commits/download-vms
The reason for the experimental stuff failing is simply that squeakci.org doesn't have curl installed (at least, if I'm reading the error message correctly).
frank