slack: fix verbose logs

By default, the desktop file of the Slack desktop client will output its
verbose logs to the system logs. To fix this, the '-s' flag which will
reduce the log level.

pkgs/applications/networking/instant-messengers/slack/default.nix

@@ -177,7 +177,8 @@ let
       # Fix the desktop link
       substituteInPlace $out/share/applications/slack.desktop \
         --replace /usr/bin/ $out/bin/ \
-        --replace /usr/share/ $out/share/
+        --replace /usr/share/ $out/share/ \
+        --replace bin/slack "bin/slack -s"
 
       runHook postInstall
     '';