Add sas private build flag

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/elara/configs/git/default.nix

@@ -1,4 +1,10 @@
-{ inputs, system, ... }:
+{
+  config,
+  inputs,
+  system,
+  lib,
+  ...
+}:
 let
   selfPkgs = inputs.self.packages.${system};
 in
@@ -22,9 +28,11 @@ in
         IdentityFile /root/.ssh/ssh_sas_ed25519_key
     '';
 
-    knownHostsFiles = with selfPkgs; [
-      ssh-known-hosts-github
-      ssh-known-hosts-sas-gitlab
-    ];
+    knownHostsFiles =
+      with selfPkgs;
+      (
+        [ ssh-known-hosts-github ]
+        ++ lib.lists.optionals config.sas.build.private [ ssh-known-hosts-sas-gitlab ]
+      );
   };
 }