If --datadir is not given, it uses '${datarootdir}/cnijlgmon3',
with the unsubstituted variable.

Motivation for this change

I have a Canon MG5620 printer which could not print using the cnijfilter2 cups driver. I found that the cnijlgmon3 program was looking for its resource file in the literal ${datarootdir}/cnijlgmon3 directory, which doesn't exist, instead of $out/share/cnijlgmon3 where the file exists.

By supplying an appropriate --datadir argument during the build, cnijlgmon3 finds its resource file, and I can print.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.