OCaml micro releases cause problems

[talex5]

When OCaml 4.10.1 was released, the PPC base image got updated before the x86 one. However, the CI assumes that the compiler version opam var from the x86 image will be the same for all arches. This meant that it tried to get the PPC 4.10.1 base image to use OCaml 4.10.0, which didn't work.

I manually redid the x86 pull, but we should probably find another way to get the variables for non-x86 platforms. One option would be to use the cluster for the query. Another might be to store the vars as labels on the image.