refactor(pixi-build-cmake): name the build directory once

[hunger]

The build script created and entered build by writing the name out, next to a template variable holding the same name and the NINJA_BUILD_DIR constant that the input tracking uses to find the tree again afterwards. Three spellings of one name, and the two in the template were already out of step: the Windows branch used the variable, while the Unix branch and the shared pushd did not.

Nothing misbehaves today, as all three say build. Changing one of them is what breaks: the input tracking would go looking for a tree the script no longer creates, and no test would notice.

The constant is now the single definition and reaches the template through the context, the way the source directory already does. The rendered scripts are unchanged.

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.

[hunger]

@Hofer-Julian This one is small and should be uncontroversial.

The next one adds a parser for cmake, that is going to be bigger (and more error prone).