Follow-up to #530: Fix broken multi-user support in `site-set` command.

This should fix the regression introduced in #530. There doesn't seem to be any reason to execute a `mkdir` every time want the temp file name, so I moved that to the only place that's actually using that information to actually *write* to the file. This also fixes a bug I discovered where `site-set @none` was emitting a success message even if it failed. (Notes in the diff.)