fix(rename): replace build.configuration with build.config

[ruben-arts]

The addition of configuration was not logical with respect to the other uses of that word in pixi so we're deprecating the configuration key.

Closes: #4035

The warning looks like:

 WARN Encountered 1 warning while parsing the manifest:
  ⚠ The `configuration` field is deprecated. Use `config` instead.
    ╭─[/Users/rubenarts/dev/pixi/examples/pixi-build/cpp-sdl/pixi.toml:34:16]
 33 │ sdl2 = ">=2.26.5,<3.0"
 34 │ [package.build.configuration]
    ·                ──────┬──────
    ·                      ╰── replace this with 'config'
 35 │ test = true
    ╰────
    ````