feat: add build number and build string prefix override options

[wolfv]

Description

This is a slight modification to teh --build-string argument. In this one we allow only modifying a prefix of the build string in order to keep the variant hash intact.

• --build-number <N>: Overrides the build number in the recipe before rendering, so the build string is automatically updated (e.g., pyh4616a5c_5 instead of pyh4616a5c_0).
• --build-string-prefix <PREFIX>: Prepends a custom string to the final build string (e.g., pr42_pyh4616a5c_0). Useful for tagging CI builds with a git hash, PR number, or branch name.
• Both flags work together: --build-number 5 --build-string-prefix pr42 → pr42_pyh4616a5c_5.

How Has This Been Tested?

I've tested it with the cpp_sdl example, eg.

cargo b --package pixi-build-cmake
export PIXI_BUILD_BACKEND_OVERRIDE=pixi-build-cmake=/Users/wolfv/Programs/pixi/target/debug/pixi-build-cmake

cargo r -- build --build-string-prefix xxx --build-number 1324
 WARN overriding build backend with: pixi-build-python=/Users/wolfv/Programs/pixi/target/debug/pixi-build-python,pixi-build-cmake=/Users/wolfv/Programs/pixi/target/debug/pixi-build-cmake
 WARN metadata cache disabled for build backend 'pixi-build-cmake' (system/path-based backends always regenerate metadata)

 ╭─ Running build for recipe: sdl_example-0.1.0-xxx_h60d57d3_1324
 │
 │ ╭─ Fetching source code
 │ │ No sources to fetch
 │ │
 │ ╰─────────────────── (took 0 seconds)
 │
 │ ╭─ Externally resolved dependencies recipe: sdl_example-0.1.0-xxx_h60d57d3_1324
 │ │
 │ │ Resolved build dependencies(sdl_example-0.1.0-xxx_h60d57d3_1324):

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

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.