fix: separate-weights build with r8.im image names fails schema validation (#2954)
When building with --separate-weights and an r8.im/ image name, the runner
image was tagged as the final image name instead of the cog-tmp:<hash>
intermediate tag. Post-build steps (schema validation, pip freeze, label
addition) then failed with 'No such image: cog-tmp:...' because the temp
tag was never created.
Fixes #2953