In the case of quri, it looks like the problem comes from the
"(asdf:clear-system c)" call at the end of "quri-test.asd". When
removing it, the build succeeds even without the custom 'asd-systems'
argument in the Guix package definition.

So it looks like the lazy loading of system definitions is working in
asdf-build-system after all. I guess it's the default behaviour of
'load-asd'.

However I don't know why this 'clear-system' call messes up the ASDF
configuration in asdf-build-system, but it doesn't seem to cause
problems in Quicklisp...