GH Actions: let actions/setup-go fetch modules automatically

This should also fix the following warning by the `actions/setup-go@v5`
action:

> Warning: Restore cache failed: Dependencies file is not found in
> /home/runner/work/kaitai_struct_go_runtime/kaitai_struct_go_runtime.
> Supported file pattern: go.sum

.github/workflows/go.yml

@@ -11,16 +11,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
+      - uses: actions/checkout@v4
+
       - name: Set up Go 1.21
         uses: actions/setup-go@v5
         with:
           go-version: '1.21'
 
-      - uses: actions/checkout@v4
-
-      - name: Get dependencies
-        run: go mod download
-
       - name: Test
         run: go test -v ./...