`--extra-experimental-features nix-command` on the command line.

* The `nix` command no longer has a syntax for referring to packages
  in a channel. This means that the following no longer works:

  ```console
  nix build nixpkgs.hello # Nix 2.3
  ```

  Instead, you can either use the `#` syntax to select a package from
  a flake, e.g.

  ```console
  nix build nixpkgs#hello
  ```

  Or, if you want to use the `nixpkgs` channel in the `NIX_PATH`
  environment variable:

  ```console
  nix build -f '<nixpkgs>' hello
  ```

* The old `nix run` has been renamed to `nix shell`, while there is a
  new `nix run` that runs a default command. So instead of

  ```console
  nix run nixpkgs.hello -c hello # Nix 2.3
  ```

  you should use

  ```console
  nix shell nixpkgs#hello -c hello
  ```

  or just

  ```console
  nix run nixpkgs#hello
  ```

  if the command you want to run has the same name as the package.

* It is now an error to modify the `plugin-files` setting via a
  command-line flag that appears after the first non-flag argument to
  any command, including a subcommand to `nix`. For example,
  `nix-instantiate default.nix --plugin-files ""` must now become
  `nix-instantiate --plugin-files "" default.nix`.

* We no longer release source tarballs. If you want to build from
  source, please build from the tags in the Git repository.

## Contributors

This release has contributions from
Adam Höse,
Albert Safin,
Alex Kovar,
Alex Zero,
Alexander Bantyev,
Alexandre Esteves,
Alyssa Ross,
Anatole Lucet,
Anders Kaseorg,
Andreas Rammhold,
Antoine Eiche,
Antoine Martin,
Arnout Engelen,
Arthur Gautier,
aszlig,
Ben Burdette,
Benjamin Hipple,
Bernardo Meurer,
Björn Gohla,
Bjørn Forsman,
Bob van der Linden,
Brian Leung,
Brian McKenna,
Brian Wignall,
Bruce Toll,
Bryan Richter,
Calle Rosenquist,
Calvin Loncaric,
Carlo Nucera,
Carlos D'Agostino,
Chaz Schlarp,
Christian Höppner,
Christian Kampka,
Chua Hou,
Chuck,
Cole Helbling,
Daiderd Jordan,
Dan Callahan,
Dani,
Daniel Fitzpatrick,
Danila Fedorin,
Daniël de Kok,
Danny Bautista,
DavHau,
David McFarland,
Dima,
Domen Kožar,
Dominik Schrempf,
Dominique Martinet,
dramforever,
Dustin DeWeese,
edef,
Eelco Dolstra,