Enable `bzlmod` for Windows + Bazel builds

The gRPC in BCR does not build on (at least) MSVC 2019. We disable `bzlmod` to work around this.

https://github.com/googleapis/google-cloud-cpp/actions/runs/9868431121

```
ERROR: C:/b/gzs2r5o4/external/grpc~~grpc_repo_deps_ext~envoy_api/envoy/extensions/load_balancing_policies/client_side_weighted_round_robin/v3/BUILD:7:18: Compiling external/grpc~~grpc_repo_deps_ext~envoy_api/envoy/extensions/load_balancing_policies/client_side_weighted_round_robin/v3/client_side_weighted_round_robin.upb_minitable.c failed: (Exit 2): cl.exe failed: error executing CppCompile command (from target @@grpc~~grpc_repo_deps_ext~envoy_api//envoy/extensions/load_balancing_policies/client_side_weighted_round_robin/v3:pkg)
```