rust-sequoia-sqv cannot be cross built from source, because its
dependency on librust-ahash-dev is not satisfiable. In general, Arch:all
packages can never satisfy cross Build-Depends unless marked
Multi-Arch: foreign or annotated :native. In this case, neither is
appropriate, because librust-ahash-dev exposes several
architecture-dependent libraries. It is a case where the multiarch
interpreter workaround definitely is needed. Please change it to
Arch:any + M-A:same as agreed earlier.

Helmut