See <https://discourse.phabricator-community.org/t/filesystem-copyfile-fails-on-windows-due-to-incompatibility-between-copy-and-bypass-shell/4161>.
`Filesystem::copyFile()` currently invokes `copy` as a subprocess, but this is a shell command and no longer works now that we use bypass_shell. See T13209.
(I haven't reproduced this yet, but the report sounds pretty credible.)
The suggested fix (use `cmd /c copy ...`) is an improvement, but using PHP `copy()` or a manual implementation might be preferable.