ATLASG-2948: Fall back to curl if TFile::Cp fails

At some point around release 25.2.60 TFile::Cp stopped copying non-ROOT files. This is completely breaks quite a few workflows (e.g. HPCs, working on local containers). This should be a fix until the issues in ATLASG-2948 are sorted out.

See also !82272 where I tried to use libcurl for this. I'm currently stuck getting CMake to find the library so the std::system call seems like a good enough stopgap.