nixos/paperless: use toShellVars for paperless-manage

The homebrewed snippet didn't escape vars properly which is an issue because
PAPERLESS_OCR_USER_ARGS requires a JSON string. This also meant a discrepancy
between the services' env vars and paperless-manage's.

Just use the correctly functioning library function for this instead.

nixos/modules/services/misc/paperless.nix

@@ -26,14 +26,11 @@ let
     lib.mapAttrs (_: toString) cfg.extraConfig
   );
 
-  manage =
+  manage = pkgs.writeShellScript "manage" ''
-    let
+    set -o allexport # Export the following env vars
-      setupEnv = lib.concatStringsSep "\n" (mapAttrsToList (name: val: "export ${name}=\"${val}\"") env);
+    ${lib.toShellVars env}
-    in
+    exec ${pkg}/bin/paperless-ngx "$@"
-    pkgs.writeShellScript "manage" ''
+  '';
-      ${setupEnv}
-      exec ${pkg}/bin/paperless-ngx "$@"
-    '';
 
   # Secure the services
   defaultServiceConfig = {