perf: Making UV the default installer, unless specified explicitly

[suleman1412]

Description

This works on making the uv the default installer in pixi_build_python crate, unless specified in the config file to be pip

Fixes #5666

How Has This Been Tested?

cargo test -p pixi_build_python

AI Disclosure

• [ x ] 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

Checklist:

• [ x ] I have performed a self-review of my own code
• [ x ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [ x ] 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.

[suleman1412]

@Hofer-Julian this PR aims to make UV the default installer in pixi_build_python unless specified explicity in the toml file