[bug#45725] Getting Gutenprint in Guix

[Faré]

For the path issue, I'm not sure how you get CUPS to see the PPDs you
want, but once it does, the remaining issue should be to rewrite some
entries in the PPD to have a full path, most importantly the
cupsFilter entries, but in the PPD from macOS there were also other
entries with paths, that may or may not apply (except maybe the last
one?):
*APDialogExtension:
*APPrinterIconPath:
*APPrinterUtilityPath:
*APPrinterLowInkTool:
*APHelpBook:
*cupsICCProfile:

Note that the NixOS recipe creates a cups-progs derivation output that
has all the ppds and all the filter binaries (themselves symlinks to
the respective derivation outputs); then it symlinks that cups-progs
output to /var/lib/cups/path and configures to look for data and
binaries there, so it doesn't have to modify any PPD.