jsonnetfile.lock.json: Update node-mixin dependency

Upgrade to prometheus/node_exporter@d574b4b, which includes a better
metric for memory saturation.

See prometheus/node_exporter#1524.

jsonnetfile.lock.json

@@ -97,7 +97,7 @@
                     "subdir": "docs/node-mixin"
                 }
             },
-            "version": "9f49fff79ef85fcebb69289622150e6d5346528b"
+            "version": "d574b4b41b72966a2ace4d6d81195081dfca8301"
         },
         {
             "name": "promgrafonnet",