nix-command doesn't change the value (always 0) of $CMD_DURATION

[demo-err]

Describe the bug

the value of $CMD_DURATION doesn't change (always 0) after performing a nix command. e.g.:

nix shell nixpkgs#figlet

Steps To Reproduce

1. do nix shell nixpkgs#boxes or any programme
2. echo $CMD_DURATION
3. try echo $CMD_DURATION after doing a command other than nix

Expected behavior

echo $CMD_DURATION should print the time it took to perform the last command

Metadata

nix-env (Nix) 2.24.14

Additional context

my default shell: fish, but i think that it's the same issue with nushell, bash, sh

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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