Use impure environment for dump command

[jgeerds]

Since /share directories of nativeBuildInputs are added to $XDG_DATA_DIRS (see direnv/direnv#443 (comment)), the original $XDG_DATA_DIRS variable (i.e. before entering the directory / loading the environment) is completely overwritten by the new one.

This just happens when using nix-direnv. The original use_nix function by direnv does not use --pure and works as expected:

use_nix() {
  direnv_load nix-shell --show-trace "$@" --run "$(join_args "$direnv" dump)"
  if [[ $# == 0 ]]; then
    watch_file default.nix shell.nix
  fi
}

This pull request fixes the issue for me.

[Mic92]

We have to --pure otherwise we cannot cache its output since it also include the users environment.

[Mic92]

Is there maybe a way to save the old value of XDG_DATA_DIRS and than just append the value from nix-shell?

[meck]

I just hit this issue, should be possible to handle XDG_DATA_DIRS the same way PATH is handled in _nix_import_env()?

[Mic92]

Solved in #71