nixos/syncthing: fix escapes interpreted in config

Dash `echo` interprets backslash escapes. This causes two consecutive backslashes in JSON to turn into a single one before the string is passed to jq, resulting in a parsing error.

nixos/modules/services/networking/syncthing.nix

@@ -46,7 +46,7 @@ let
     old_cfg=$(curl ${cfg.guiAddress}/rest/config)
 
     # generate the new config by merging with the NixOS config options
-    new_cfg=$(echo "$old_cfg" | ${pkgs.jq}/bin/jq -c '. * {
+    new_cfg=$(printf '%s\n' "$old_cfg" | ${pkgs.jq}/bin/jq -c '. * {
         "devices": (${builtins.toJSON devices}${optionalString (! cfg.overrideDevices) " + .devices"}),
         "folders": (${builtins.toJSON folders}${optionalString (! cfg.overrideFolders) " + .folders"})
     } * ${builtins.toJSON cfg.extraOptions}')