ci: test using Node.JS v18.0

This would have caught
like braintree#78, as
`URL.canParse()` was only added in Node.JS v18.17

.github/workflows/ci.yml

@@ -12,6 +12,6 @@ jobs:
       - name: Use Node.js
         uses: actions/setup-node@v1
         with:
-          node-version: "18.x"
+          node-version: "18.0"
       - run: npm install
       - run: npm test