jsonnet: pin node-mixin due to upstream bug

jsonnetfile.json

@@ -8,6 +8,16 @@
         }
       },
       "version": ""
+    },
+    {
+      "name": "node-mixin",
+      "source": {
+        "git": {
+          "remote": "https://github.com/prometheus/node_exporter",
+          "subdir": "docs/node-mixin"
+        }
+      },
+      "version": "5a7b85876d6108a91f0d8673c0d7eca38687671b"
     }
   ]
 }