Seen in 4390954027:
nix-functional-tests> grep: repl_output: No such file or directory
nix-functional-tests> +(repl.sh:174) cat repl_output
This is because there is a small possibility that the `nix repl` child
process hasn't created `repl_output` yet. So make sure it exists.
(cherry picked from commit 9eb46e9cc0)