Using `configure_file` to copy files has been deprecated since Meson 0.64.0.
The intended replacement is the `fs.copyfile` method.
This removes the following deprecation warning that arises when a minimum
Meson version is specified:
``
Project [...] uses feature deprecated since '0.64.0': copy arg in configure_file. Use fs.copyfile instead
``
Change-Id: I09ffc92e96311ef9ed594343a0a16d51e74b114a
Allows completing `nix build ~/flake#<Tab>`.
We can implement expansion for `~user` later if needed.
Not using wordexp(3) since that expands way too much.
Make nix output completions in the form `completion\tdescription`.
This can't be used by bash (afaik), but other shells like zsh or fish
can display it along the completion choices