nixos/stage-2-init: dont use install to create /etc/nixos if it's a symlink

it should be checking that it is not a broken symlink but bash
conditionals are difficult

-d was causing the directory to not be created if it does not exist

```
$ install -m 0755 -d $PWD/hello
$ ls
hello/
$ ln -s something notexist
'notexist' -> 'something'
$ ls -l
lrwxrwxrwx artturin artturin 9 B Sat Sep  9 06:59:44 2023 notexist@ ⇒ something
drwxr-xr-x artturin artturin 2 B Sat Sep  9 06:59:36 2023 hello/
$ install -m 0755 -d $PWD/notexist
install: cannot change permissions of ‘/home/artturin/nixgits/my-nixpkgs/test/notexist’: No such file or directory
```

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

@@ -105,7 +105,7 @@ fi
 
 # Required by the activation script
 install -m 0755 -d /etc
-if [ -d "/etc/nixos" ]; then
+if [ ! -h "/etc/nixos" ]; then
     install -m 0755 -d /etc/nixos
 fi
 install -m 01777 -d /tmp