rust:1.88-slim (Debian Bookworm) uses DEB822-format sources which have no arch
restriction. After dpkg --add-architecture arm64, apt tries to resolve deps for
both amd64 and arm64 simultaneously and hits 'held broken packages' conflicts on
shared -dev packages.
Fix: remove debian.sources and write a clean sources.list that pins amd64 repos
to [arch=amd64] and arm64 repos to [arch=arm64]. This gives apt a clear,
non-conflicting view of each architecture's package set.