Update CI

Update actions/checkout to v2

Author: lovasoa

.github/workflows/CI.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - uses: mymindstorm/setup-emsdk@b6b33c4
       with: {version: '1.39.11'}
     - uses: actions/cache@v1