I'm testing this on a Mac (nix flakes enabled), and I'm getting this output:

% nix develop
mktemp: illegal option -- -
usage: mktemp [-d] [-q] [-t prefix] [-u] template ...
       mktemp [-d] [-q] [-u] -t prefix
### 🔨 Welcome to devshell ####
[commands]
devshell-menu - print this menu
devshell-root - change directory to root
[devshell]$ 

I've been unable to find any mention of mktemp in this repo, however it seems to be related to mkDevShell since I can use mkShell from nixpkgs without ever seeing the above output. On NixOS everything is fine.