nixos/promtail: Replace custom jq-based runCommand with pkgs.formats

nixos/modules/services/logging/promtail.nix

@@ -2,9 +2,11 @@
 let
   cfg = config.services.promtail;
 
-  prettyJSON = conf: pkgs.runCommandLocal "promtail-config.json" {} ''
-    echo '${builtins.toJSON conf}' | ${pkgs.buildPackages.jq}/bin/jq 'del(._module)' > $out
-  '';
+  format = pkgs.formats.json {};
+  prettyJSON = conf: with lib; pipe conf [
+    (flip removeAttrs [ "_module" ])
+    (format.generate "promtail-config.json")
+  ];
 
   allowSystemdJournal = cfg.configuration ? scrape_configs && lib.any (v: v ? journal) cfg.configuration.scrape_configs;
 
@@ -20,7 +22,7 @@ in {
     enable = mkEnableOption "the Promtail ingresser";
 
     configuration = mkOption {
-      type = (pkgs.formats.json {}).type;
+      type = format.type;
       description = ''
         Specify the configuration for Promtail in Nix.
         This option will be ignored if `services.promtail.configFile` is defined.