Runner builds suddenly can't find gitlab-runner-prebuilt.tar.xz

My builds started failing with this error:

ERROR: Failed to create container volume for /cache open /var/lib/gitlab-runner/gitlab-runner-prebuilt.tar.xz: no such file or directory
ERROR: Failed to create container volume for /builds/david open /var/lib/gitlab-runner/gitlab-runner-prebuilt.tar.xz: no such file or directory
ERROR: Build failed (system failure): open /var/lib/gitlab-runner/gitlab-runner-prebuilt.tar.xz: no such file or directory

I've got the following packages installed on Debian/testing:

ii  gitlab-ce                                                       8.10.7-ce.0                          amd64                                GitLab Community Edition and GitLab CI (including NGINX, Postgres, Redis)
ii  gitlab-ci-multi-runner                                          1.4.2+dfsg-1                         amd64                                GitLab Runner

It was working 2 weeks ago, and believe there was a package updated between then and now.

I don't understand where gitlab-runner-prebuilt.tar.xz comes from or why it might be missing. Any idea how to fix this?