nix-direnv: fix NIX_BIN_PREFIX substitution in built direnvrc
nix-direnv: fix NIX_BIN_PREFIX substitution in built direnvrc

* Reason: statically set `${NIX_BIN_PREFIX}` within generated
  `direnvrc`.
* Before it only replaced the `-z ${NIX_BIN_PREFIX:-}` check
  which therefore never was `true`. So, also `NIX_BIN_PREFIX` never got
  set such that its usage later always failed execution with
  `NIX_BIN_PREFIX: unbound variable`. With the fix, the line containing
  the check will no longer be replaced.
* Solves https://github.com/nix-community/nix-direnv/issues/70
* See also discussion on #114622

Signed-off-by: Andreas Schmid <4cf5bc59bee9e1c44c6254b5f84e7f066bd8e5fe@aaschmid.de>

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