Enable repl-flake experimental feature

[Hoverbear]

Description

The repl-flake feature is really handy for folks adopting Nix with flakes.

Nix already warns about needing to use -f or --file in the future for the 'old' behavior:

❯ nix repl flake.nix
warning: future versions of Nix will require using `--file` to load a file

This makes stuff like the following work:

❯ nix repl .#
Welcome to Nix 2.17.0. Type :? for help.

Loading installable 'git+file:///home/ana/git/determinatesystems/nix-installer#'...
Added 5 variables.
nix-repl> packages
{ aarch64-darwin = { ... }; aarch64-linux = { ... }; i686-linux = { ... }; x86_64-darwin = { ... }; x86_64-linux = { ... }; }

And:

❯ nix repl "github:nixos/nixpkgs?dir=lib"
Welcome to Nix 2.17.0. Type :? for help.

Loading installable 'github:nixos/nixpkgs?dir=lib#'...
Added 1 variables.
nix-repl> lib.
lib.__unfix__                      lib.debug                          lib.genList                        lib.isType                         lib.mesonEnable                    lib.pathIsDirectory                lib.testAllTrue
lib.add                            lib.deepSeq
# ...

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

noice