Add `target-store` and `eval-store` settings

[thufschmitt]

Is your feature request related to a problem? Please describe.

Confusion around --store like in #10119

Describe the solution you'd like

Cleanly separate the build and evaluation stores in the interface. For that

• Promote the existing eval-store CLI option to a parameter
• Add a new target-store parameter for the store used to build things
• Make store set defaults for both eval- and target-

Describe alternatives you've considered

Additional context

In the longer run, we'll also want to split build-store (or build-stores) out of target-store, which would solve #6893 and give Nix the same model as Hydra.

Priorities

Add 👍 to issues you find important.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-03-04-nix-team-meeting-minute-130/40830/1

[roberth]

I think I prefer destination or output instead of target, because build and target sounds like we're doing cross compilation.

What happens to the result symlink?

[thufschmitt]

Yup' destination is better, indeed (and more coherent with the rest of Nix)

What happens to the result symlink?

If the target store isn't a LocalFSStore, it isn't created (already the case on master)

[roberth]

output would imply that we don't copy the build dependencies there. It'd be equivalent to doing a build anywhere and copying only the outputs to the --output-store store.

For destination it wouldn't be as unexpected for the paths there to be a mix of actual final outputs and outputs of dependencies that were used during the build (which may not be dependencies of the requested outputs).