gha: update to actions/checkout@v3

Author: MisterDA

.github/workflows/main.yml

@@ -25,7 +25,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Get latest OCaml commit hash
         id: multicore_hash