feat(incusos): build and publish multiple variants

[jmgilman]

Summary

• build all configured IncusOS variants in deterministic variant-name order
• pass artifact variants through imgsrv catalog publication and publish JSON
• adapt publish to wait for imgsrv durable publish jobs

Verification

• proto run go -- test -count=1 ./internal/providers/incusos ./internal/publish ./internal/cli
• proto run go -- test -count=1 -tags integration ./cmd/imgcli
• moon run root:check --summary minimal