Pass coreos-assembler image by digest when triggering build-arch job

[jlebon]

Idea from @jschintag. It's unlikely the cosa image we used in the x86_64 image has already been GC'ed, so we should be able to pass the actual sha256 :main resolved to when triggering build-arch from build.

[dustymabe]

It's unlikely the cosa image we used in the x86_64 image has already been GC'ed,

I believe in quay as soon as a tag gets updated the old version isn't available (but it is available to restore via "time machine" in the web interface).