nixos/sane: point SANE_CONFIG_DIR away from /etc/sane.d

This brings back the ability to e.g. configure sane-airscan with
`environment.etc."sane.d/airscan.conf".text = ...`.

(AFAICT, sane-airscan loads all config files it finds, so it'll first
load the one from the nixos hardware.sane.* configuration, then the user
specified one in /etc/sane.d/airscan.conf.)

Fixes: 4fbec87a5bfc ("nixos/sane: point env vars to /etc for quick reload")

Fixes https://github.com/NixOS/nixpkgs/issues/207262

nixos/modules/services/hardware/sane.nix

···
   };
 
   env = {
-    SANE_CONFIG_DIR = "/etc/sane.d";
+    SANE_CONFIG_DIR = "/etc/sane-config";
     LD_LIBRARY_PATH = [ "/etc/sane-libs" ];
   };
 
···
 
       environment.systemPackages = backends;
       environment.sessionVariables = env;
-      environment.etc."sane.d".source = config.hardware.sane.configDir;
+      environment.etc."sane-config".source = config.hardware.sane.configDir;
       environment.etc."sane-libs".source = "${saneConfig}/lib/sane";
       services.udev.packages = backends;