f9883482d made a login id name one file, but on Linux a backslash is an
ordinary character, so `..\evil` passed as a local name and
TestSaveAccountRejectsTraversalID has failed the ubuntu, macOS and race
jobs on main-v2 since. The same config directory can be read on Windows,
where that name climbs out of the account folder, so both separators are
refused everywhere.