This replicates behaviour found in the Nix2 commands, where IN_NIX_SHELL
is set. This is for shells to determine whether they are inside of a
Nix3 shell, and set a custom prompt accordingly.
For example, Fish's Tide prompt framework checks for that environment
variable and displays it in the prompt, indicating that the shell is in
a Nix environment.
This is not new behaviour, and the old Nix2 commands set the variable.
If the shell that is created is a "pure" shell, ie --ignore-environment
is passed, then IN_NIX_SHELL will be set to "pure". However, "nix
develop" will always create an impure environment.
Replicated from my Nix PR: https://github.com/NixOS/nix/pull/8885
Change-Id: I695cdc336f76541940a302835124fe7d8f7f39b2
Signed-off-by: Ersei Saggi <vcs@ersei.net>
