Respect existing _nix_direnv_manual_reload

[JoaquinTrinanes]

It's only initialized to 0 if it's not already set.

This fixes a niche-ish direnv use-case: I have nix_direnv_manual_reload set globally (inside my direnvrc config file). It works, but if I have nested envfiles it will be disabled (unless specified again). This can also be triggered by having nix_direnv_manual_reload on the nested .envrc but not on the outer one.

MWE

cd "$(mktemp -d)"

echo $'
{
  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
  outputs =
    { nixpkgs, ... }:
    let
      forEachSystem =
        f: nixpkgs.lib.genAttrs nixpkgs.lib.systems.flakeExposed (system: f nixpkgs.legacyPackages.system);
    in
    {
      devShells = forEachSystem (pkgs: {
        default = pkgs.mkShell { };
      });
    };

}' >flake.nix
echo 'use flake' >.envrc

mkdir inner
echo $'
nix_direnv_manual_reload
source_up
' >./inner/.envrc
cd inner
direnv allow # manual reload not set

[bbenne10]

This LGTM, but have not tested. Generally unavailable until Monday, so may not test until then.