Incomplete privilege drop for com.system76.CosmicGreeter.GetUserData

A Privilege Dropping / Lowering Errors/Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability in  cosmic-greeter can allow an attacker to regain privileges that should have been dropped and abuse them in the racy checking logic. This issue affects cosmic-greeter before https://github.Com/pop-os/cosmic-greeter/pull/426.