Cannot the hardcoded archictecture list be avoided with the (virtual)
packages architecture-is-{little-endian,64-bit}?
You cannot use these virtual packages in Architecture.
You cannot use these virtual packages for conditional build dependencies.
For autopkgtest skip-not-installable is the best option to avoid the
hardcoding.
src:cantor from the key package path in this discussion:
Source: cantor
Build-Depends: ...,r-base-core,...
...
Package: cantor-backend-r
Architecture: any
That's two places where architecture lists will have to be hardcoded
if r-base-core would drop 32-bit support (which might not yet happen
as part of the current change).