jsonnet: pin node-mixin due to upstream bug

jsonnetfile.json

 {
-    "dependencies": [
-        {
-            "name": "kube-prometheus",
-            "source": {
-                "local": {
-                    "directory": "jsonnet/kube-prometheus"
-                }
-            },
-            "version": ""
+  "dependencies": [
+    {
+      "name": "kube-prometheus",
+      "source": {
+        "local": {
+          "directory": "jsonnet/kube-prometheus"
         }
-    ]
+      },
+      "version": ""
+    },
+    {
+      "name": "node-mixin",
+      "source": {
+        "git": {
+          "remote": "https://github.com/prometheus/node_exporter",
+          "subdir": "docs/node-mixin"
+        }
+      },
+      "version": "5a7b85876d6108a91f0d8673c0d7eca38687671b"
+    }
+  ]
 }