nitrokey: Force learn keys from card on plug

This allows the signing key to be shared among multiple nitrokeys.

modules/nitrokey.nix

@@ -15,5 +15,15 @@ lib.mkMerge [
       #nitrokey-app2
       #pynitrokey
     ];
+
+    systemd.user.services.gpg-force-learn-cards = {
+      serviceConfig = {
+        ExecStart = "${pkgs.gnupg}/bin/gpg-connect-agent 'learn --force' '/bye'";
+      };
+    };
+
+    services.udev.extraRules = ''
+      SUBSYSTEM=="usb", ACTION=="add", ATTRS{idVendor}=="20a0", ATTRS{idProduct}=="42b2", ENV{SYSTEMD_USER_WANTS}="gpg-force-learn-cards.service"
+    '';
   })
 ]