nix-repl> content = builtins.readFile ./test

nix-repl> content
"Content: ${x}\n"

nix-repl> builtins.tryEval "${content}"
{ success = true; value = "Content: ${x}\n"; }

nix-repl> x = "123"

nix-repl> builtins.tryEval "${content}"
{ success = true; value = "Content: ${x}\n"; }