As described in detail here: https://github.com/NixOS/nixpkgs/issues/32533
bash will load completion scripts in $p/share/bash-completion/completions/ on
startup instead of letting bash-completion do it's lazy loading. Bash startup
will then slow down (very noticeable when bash-completion is installed in a
profile).
This commit leaves loading of scripts in the hands of bash-completion,
improving startup time for everyone using `enableCompletion`.
Fixes#101
This should enable `nix run` to work under shells like fish and zsh,
as well as making child shells not needlessly reset any environment
that should be inherited.
Implementation adapted from NixOS.
