Debugging missing architecture support

[Konrad Hinsen]

Hi Guix experts,

in trying to build a Docker image for ARM64 (aarch64-linux), I am
hitting errors of the kind

  "package git-annex@10.20230926 does not support aarch64-linux"

There doesn't seem to be anything specifically in the package
definitions that precludes building for aarch64-linux, so I suspect it's
either a dependency or the build system that lacks the required support.

How would I go about debugging such issues? Maybe the causes are simple
and I can fix them. But I don't even know where to start.

Cheers,
  Konrad.