Fix filename for heftig's new key in root_access

The new key is under pubkeys/heftig_nitrokey.pub, and pubkeys/heftig.pub
was renamed to pubkeys/heftig_yubikey.pub. Update root_ssh_keys to refer
to "heftig_nitrokey.pub" as "heftig.pub" does not exist anymore.

Fixes: 13a4bddf ("pubkeys/heftig: Add my new Nitrokey, reorganize")

group_vars/all/root_access.yml

@@ -16,7 +16,7 @@ root_ssh_keys:
   - key: foutrelis.pub
   - key: freswa.pub
   - key: grazzolini.pub
-  - key: heftig.pub
+  - key: heftig_nitrokey.pub
   - key: jelle.pub
   - key: svenstaro.pub
   - key: anthraxx.pub