The intention of this change is to mute error messages caused when multiple users use nix-shell, but only the first one succeeds to dump their environment variables. All others face a permission denied error like this:

$ nix-shell -p coreutils
bash: /tmp/env-vars: Permission denied

[nix-shell:~]$ 

This might not be the most sensible way to resolve this issue, but the fix was inspired by the irrelevance of the dumping's success, i.e. || true... ;)