Runner failing to pull correct image

This comes from a conversation with lavamind on IRC.

See the following failure that suddenly started happening: https://gitlab.torproject.org/vcrn/arti/-/jobs/896879

A similar issue is also impacting one of our night builds: see job: https://gitlab.torproject.org/tpo/core/arti/-/jobs/896686 from the matrix check in https://gitlab.torproject.org/tpo/core/arti/-/pipelines/259118