replace latest with v4 in CI

Author: jbytecode

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
         arch:
           - x64
     steps:
-      - uses: actions/checkout@latest
+      - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v2
         with:
           version: ${{ matrix.version }}