Try exact stable node 8 versions.

opentech/static_src/package.json

@@ -97,7 +97,7 @@
     "sass-lint": "^1.12.1"
   },
   "engines": {
-    "node": ">=7.0.0"
+    "node": "8.11.3"
   },
   "dependencies": {
     "@fancyapps/fancybox": "^3.2.5"

package.json

@@ -4,9 +4,10 @@
     "description": "Build the static src.",
     "cacheDirectories": ["client/node_modules"],
     "scripts": {
-      "postinstall": "cd opentech/static_src && yarn install && npm build:prod"
+        "postinstall": "cd opentech/static_src && yarn install && npm run build:prod"
     },
     "engines": {
-    "yarn": ">=1.7.0"
+        "node": "8.11.3",
+        "yarn": "1.7.0"
     }
 }