fix: put .pixi/build in detached-environments location

[moritzwilksch]

Description

This changes pixi to store the .pixi/build directory alongside the environment in the detached location when detached-environments is enabled.

I'm using detached-environments because my project directory lives on a slow network filesystem, so I create the pixi environment in a faster, machine-local location. That works well for the environment itself, but .pixi/build still gets created in the project directory, which slows things down. It would be more consistent to put that directory in the detached location too.

Fixes #4320

How Has This Been Tested?

1. git clone https://github.com/moritzwilksch/pixi-workspace-demo && cd pixi-workspace-demo
2. Set detached-environments="/tmp" in ~/.pixi/config.toml
3. Build pixi on this branch
4. pixi install
5. ls -l .pixi/

➜  pixi-workspace-demo git:(main) ✗ /Users/moritzwilksch/Documents/pixi/target/debug/pixi i
✔ The default environment has been installed in '/tmp'.
➜  pixi-workspace-demo git:(main) ✗ ll .pixi/
total 0
lrwxr-xr-x@ 1 moritzwilksch  staff    42B Mar 27 16:23 build@ -> /tmp/wspace-demo-7225176843553100987/build
lrwxr-xr-x@ 1 moritzwilksch  staff    41B Mar 27 16:23 envs@ -> /tmp/wspace-demo-7225176843553100987/envs
➜  pixi-workspace-demo git:(main) ✗

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude, Codex

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[pavelzw]

sorry, forgot to review this after dealing with the exclude-newer stuff. @ruben-arts solved it in #5855 so i guess we can close this one 😅

[ruben-arts]

Oh sorry this also completely missed my scope 🥲