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

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

src/util/system.cpp

@@ -146,7 +146,7 @@ bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes)
 }
 
 std::streampos GetFileSize(const char* path, std::streamsize max) {
-    std::ifstream file(path, std::ios::binary);
+    fsbridge::ifstream file{path, std::ios::binary};
     file.ignore(max);
     return file.gcount();
 }