Update post-install script

Signed-off-by: Severin Kaderli <severin@kaderli.dev>

system/.config/i3/config.base

@@ -69,6 +69,7 @@ assign [class="Nemo"] $workspace4
 assign [class="Thunar"] $workspace4
 assign [class="discord"] $workspace5
 assign [class="Riot"] $workspace5
+assign [class="Slack"] $workspace5
 assign [class="Steam"] $workspace6
 assign [class="keepassxc"] $workspace8
 assign [class="vlc"] $workspace9