nixos/stage-2-init: Change permission of /etc/nixos only if it exists

When I boot there's a warning `stage-2-init: install: cannot change permissions of '/etc/nixos': No such file or directory`
because my /etc/nixos is a symlink to $HOME/dotfiles.

```
/etc/nixos -> /home/artturin/dotfiles
```

These lines were added in 56b4653904

nixos/modules/system/boot/stage-2-init.sh

@@ -104,7 +104,10 @@ fi
 
 
 # Required by the activation script
-install -m 0755 -d /etc /etc/nixos
+install -m 0755 -d /etc
+if [ -d "/etc/nixos" ]; then
+    install -m 0755 -d /etc/nixos
+fi
 install -m 01777 -d /tmp