Update main.yml workflow to use actions/checkout@v2

Author: SRombauts

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Submodule SQLiteCpp
       run: |
         git submodule init