Hi!

I'm not sure if the following suggestion is a canonical way of doing
things (I'm a scheme newb), but it is possible to define a package variant
of diffoscope with a G-expression that uses wrap-program (see
guix/build/utils.scm) to include paths to dependencies as part of an
environment variable.

From the docstring:

"This is useful for scripts that expect particular programs to be in
$PATH, for programs that expect particular shared libraries to be in
$LD_LIBRARY_PATH, or modules in $GUILE_LOAD_PATH, etc."

As Liliana said, it is possilbe to iterate over package-inputs to compute
the paths in the package definition of a diffoscope variant. So until the
problem with xb-tool is fixed, maybe this is a good way to go?