gnucash: Add libdbiDrivers env var to wrapper

The GNU Cash package clearly *wants* to be able to use the libdbi
backends, since it pulls them in as dependencies.  However, you can only
open xml formatted GNU cash files.

The CMake scripts hard-code the DVD path to be basically
<PATH_OF_LIBDBI>/dbd.  However GNU Cash does check the environment
variable GNC_DBD_DIR, so I set that environment variable in the wrapper
script.

With this change, you should be able to e.g. "Save As" in the sqlite
format.

Fixes issue #57445

pkgs/applications/office/gnucash/default.nix

@@ -65,6 +65,7 @@ stdenv.mkDerivation rec {
       --prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH:$out/share/gsettings-schemas/${pname}-${version}" \
       --prefix XDG_DATA_DIRS : "${hicolor-icon-theme}/share" \
       --prefix PERL5LIB ":" "$PERL5LIB" \
+      --set GNC_DBD_DIR ${libdbiDrivers}/lib/dbd \
       --prefix GIO_EXTRA_MODULES : "${stdenv.lib.getLib dconf}/lib/gio/modules"
   '';