AMD64 build still broken

https://arm01.puri.sm/job/debs/job/deb-squeekboard-buster+ci-amd64/82/console

compare https://arm01.puri.sm/job/debs/job/deb-squeekboard-buster+ci-aarch64/86/console

The culprit is a missing call to rm Cargo.lock. This seems to be caused by arm64 calling debian/rules build, and amd64 calling debian/rules build-arch, which is not filled in.

@guido.gunther how to deal with it? Is the build-arch targt going to be called by build? Are they both independent? Why are different targets called anyway?