ci: use node 8 in appveyor

Workaround for nodejs/node#20297

Author: filipesilva

.appveyor.yml

@@ -1,5 +1,7 @@
 environment:
-  nodejs_version: "10"
+  # Note: if updating to Node 10, use at least 10.5.0 to include a fix for
+  # https://github.com/nodejs/node/issues/20297
+  nodejs_version: "8.9.2" # Same version as used in CircleCI.
 
 matrix:
   fast_finish: true