check isFlake in nixpkgsFlakeRef

[KiaraGrouwstra]

Add a check for isFlake in nixpkgsFlakeRef.

Motivation

To get bashInteractive, nix develop currently gets a flake reference to nixpkgs, either from inputs.nixpkgs or flake registry entry nixpkgs.

Currently, this is done by name for any (locked) reference. For any nixpkgs reference lacking a flake.nix, nix develop thus errors:

error (ignored): path '.../flake.nix' does not exist

Now, given a inputs.<name>.flake = false;, we may know in advance that our reference should not be a reference to a flake.

This change incorporates this knowledge, so that given inputs.nixpkgs.flake = false;, nix develop will fall back to flake registry entry nixpkgs to use as its flake for bashInteractive.

While Nixpkgs itself of course does expose a flake, this check is relevant in particular for dummy inputs (e.g. inputs using { url = "file:/dev/null"; flake = false; }), which may be overridden by --override-input or .follows, yet do not expose (flake) code themselves.

Context

• Note that this change takes at face value the name nixpkgsFlakeReference, which reflects current use of the function, although the fetch-tree experimental feature has broadened the use of what were historically called flake references beyond use to refer to flake-enabled source trees.
• I made an example for such use of dummy inputs at https://codeberg.org/kiara/poc-override-nix-deps.

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[roberth]

Getting bashInteractive seems a bit useless since

• bash: Make interactive by default nixpkgs#379368

Furthermore none of this will be necessary when we implement

• Allow to get rid of nix develop "shell" logic #7501

That said, since we don't have that yet, I would suggest to go ahead with this change, which is good.