Add sas private build flag

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

hosts/elara/options.nix

@@ -0,0 +1,9 @@
+{ lib, ... }:
+with lib;
+{
+  options.sas.build.private = mkOption {
+    type = types.bool;
+    description = "Whether to build private SAS derivations (must be connected to the company VPN).";
+    default = false;
+  };
+}