Escape ${ in strings when printing Nix expressions

Otherwise the result of the printing can't be parsed back correctly by
Nix (because the unescaped `${` will be parsed as the begining of an
anti-quotation).

Fix #3989

tests/user-envs.nix

@@ -13,7 +13,7 @@ let
     builder = ./user-envs.builder.sh;
   } // {
     meta = {
-      description = "A silly test package";
+      description = "A silly test package with some \${escaped anti-quotation} in it";
     };
   });