CI: Use checkout@v2 and setup-node@v2 (#617)

- actions/checkout v2 is faster by doing a shallow clone
- actions/setup-node v2 improves reliability

.github/workflows/build.yml

@@ -16,10 +16,10 @@ jobs:
 
     steps:
       - name: Check out Git repository
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Install Node.js, NPM and Yarn
-        uses: actions/setup-node@v1
+        uses: actions/setup-node@v2
         with:
           node-version: 12