chore: lib.types.string -> lib.types.str

lib.types.string concatenates conflicting definitions on module merge
which is counter intuitive and hard to debug behavior.

Change-Id: I8ccdca0e8895fb5cc4600c367f8d52a9ab80ff75
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13177
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: aspen <root@gws.fyi>

users/aspen/xanthous/server/module.nix

@@ -27,7 +27,7 @@ in
       };
 
       ed25519SecretKeyFile = mkOption {
-        type = with types; uniq string;
+        type = with types; uniq str;
         description = "Path to the ed25519 secret key for the server";
       };
     };