nixos/cupsd: Set CUPS_DATADIR globally

This is used by some programs that need CUPS data files. For instance,
print-manager looks here for printing test pages.

nixos/modules/services/printing/cupsd.nix

@@ -296,6 +296,10 @@ in
     # gets loaded, and then cups cannot access the printers.
     boot.blacklistedKernelModules = [ "usblp" ];
 
+    # Some programs like print-manager rely on this value to get
+    # printer test pages.
+    environment.sessionVariables.CUPS_DATADIR = "${bindir}/share/cups";
+
     systemd.packages = [ cups.out ];
 
     systemd.sockets.cups = mkIf cfg.startWhenNeeded {