ci: cache go modules from core/go.sum, retry downloads

.github/workflows/prek.yml

@@ -26,6 +26,11 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: core/go.mod
+          cache-dependency-path: core/go.sum
+
+      - name: Download Go modules
+        working-directory: core
+        run: for attempt in 1 2 3; do go mod download && exit 0; sleep 15; done; exit 1
 
       - name: run pre-commit hooks
         uses: j178/prek-action@v2