Run ember-try scenarios in Node 10.

This would have caught the error reported in
emberjs/ember.js#19431, we should run with the
lowest supported Node version.

Author: rwjblue

.github/workflows/ci.yml

@@ -89,7 +89,7 @@ jobs:
     - uses: actions/checkout@v1
     - uses: actions/setup-node@v1
       with:
-        node-version: 12.x
+        node-version: 10.x
     - name: install dependencies
       run: yarn install
     - name: test