fix(binary): conditional cosign install for non-catalog build_image
v2.0.0 image-cutover removed cosign install assuming catalog-image adoption. Consumer migrations surfaced that non-catalog build_image values (rust:1.88.0, golang:1.25) lose cosign. Conditional install restores v1.x consumer compatibility.
Strict relaxation: catalog-image consumers are unaffected.
After merge → v2.1.1 → consumer pin updates.