This caused concurrent writers to operate on the same file, leading to file corruption. In some cases, this manifested as test failures and intermittent std::bad_alloc exceptions.
Change Description
This change ensures that each test instance uses a unique filename for downloaded bucket files.
By isolating file writes per test execution, concurrent runs no longer interfere with each other.
Fixes: #27824
backport not required
Closesscylladb/scylladb#27843