GitHub Actions: use actions/checkout@v2 (#563)

Author: StefH

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: windows-latest
 
     steps:  
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
 
     - name: Build Projects
       run: |