ssh-agent: fix syntax problem from #71139
ssh-agent: fix syntax problem from #71139

Oops, in #71139 a missing `+` broke things quite badly.  Thanks @lzorkin for the
report and @mebubo for diagnosing the problem.
NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs