launchd: Restore errexit after setting up launchd agents

At the beginning of the setup script `errexit` is disabled, but never
properly enabled again at the end. This causes potential issues/errors
in activation scripts following `setupLaunchAgents` to go unnoticed,
since the build doesn't actually fail.

modules/launchd/default.nix

@@ -270,9 +270,7 @@ in
             setupLaunchAgents
 
             # Restore errexit
-            if [[ -o errexit ]]; then
-              set -e
-            fi
+            set -e
           '';
     })
   ];