Remove action pin

Author: dspinellis

.github/workflows/main.yml

@@ -16,7 +16,7 @@ jobs:
         os: [ubuntu-latest, macOS-latest]
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@main
 
     - name: Install dependencies (Ubuntu)
       if: runner.os == 'Linux'