Android cross environment isn't fully delocalised

### Describe the bug

The `xvenv` environment created for Android contains definitions for CC and other build tools that reference the full path to `ANDROID_HOME` on the machine where Python was built.

### Steps to reproduce

1. Create and activate an Android xvenv
2. Run `python -c "import sysconfig; print(sysconfig.get_config_var('CC'))"
3. See a path like `/Users/msmith/Library/Android/sdk/ndk/27.2.12479018/toolchains/llvm/prebuilt/darwin-x86_64/bin/aarch64-linux-android21-clang`, even if you're not msmith.

### Expected behavior

Portions of the sysconfig file that include references to the build machine (especially `ANDROID_HOME`) should be delocalised.

### Screenshots

_No response_

### Environment

- Operating System: Any (but targing Android)
- Python version: 3.13.5
- Software versions:
- xbuild: 0.2.0

### Logs

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### Additional context

`cibuildwheel` avoids this by invoking `android_env.sh` to set variables in the Android environment. It may be appropriate for `xbuild` to do the same - although it's unclear where that environmental configuration would be applied.

Start by tracing how the Android xvenv generates its sysconfig definitions and how the build-machine ANDROID_HOME path enters CC and related build-tool settings. Compare the approach with cibuildwheel's android_env.sh handling; done means an Android xvenv no longer embeds machine-specific paths, including the reported CC value.

Written by the indexing model from the issue text.