Is your feature request related to a problem?

If a build fails, it is very useful to be able to enter the build environment and troubleshoot it interactively. As far as I can tell, the only way to do this right now is by adding nixpkgs' special package breakpointHook to the nativeBuildInputs of a troublesome derivation. Doing this may be quite difficult, especially for inexperienced Nixpkgs developers, and it also perturbs the build environment in a way that could potentially mask the problem (since there are additional packages visible to the build).

The nix3 build subcommand has an option --debugger which allows interactive debugging of a failed Nix expression evaluation, but this doesn't help with a failed build, and it's not, as far as I can tell, available to traditional nix commands.

Proposed solution

1. Add the --debugger option to all the traditional nix commands.
2. Expand the functionality of --debugger to include an effect akin to the effect of adding breakpointHook to nativeBuildInputs, but, crucially, without requiring modification of the derivation or perturbing the build environment.

Checklist

• checked latest Nix manual (source) [Note: I could not find the "new-cli" manpages in the source tree]
• checked open feature issues and pull requests for possible duplicates
• reviewed the contributing guide

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