fs: consistently use fsbridge for {i,o}fstream

Part of #20744, but this can be done now, and will simplify the diff.

src/test/fuzz/fuzz.cpp

@@ -80,7 +80,7 @@ void initialize()
     }
     if (const char* out_path = std::getenv("WRITE_ALL_FUZZ_TARGETS_AND_ABORT")) {
         std::cout << "Writing all fuzz target names to '" << out_path << "'." << std::endl;
-        std::ofstream out_stream(out_path, std::ios::binary);
+        fsbridge::ofstream out_stream{out_path, std::ios::binary};
         for (const auto& t : FuzzTargets()) {
             if (std::get<2>(t.second)) continue;
             out_stream << t.first << std::endl;