nix: add -U option to gnatprove

Signed-off-by: Thomas Heijligen <src@posteo.de>
diff --git a/default.nix b/default.nix
index 0ce641b..d7a6d66 100644
--- a/default.nix
+++ b/default.nix
@@ -22,7 +22,7 @@
     runHook preBuild
 
     gprbuild -P ${gprFile}
-    gnatprove -P ${gprFile} --prover=z3
+    gnatprove -P ${gprFile} -U --prover=z3
 
     runHook postBuild
   '';