perf: split multi-arch builds to native runners (no QEMU)

Replace single QEMU-emulated multi-arch build with parallel native
builds on ubuntu-latest (amd64) and ubuntu-24.04-arm (arm64), then
merge manifests. Expected build time: ~15-20 min instead of ~90+ min.

- build-amd64: native x86_64 on ubuntu-latest
- build-arm64: native ARM64 on ubuntu-24.04-arm
- merge: create multi-arch manifest, sign, scan, test
- Default timeout reduced from 180 to 60 minutes
- No caller workflow changes needed

Author: sylvesterdamgaard