Use sys.base_exec_prefix for Python_ROOT_DIR

`Python_ROOT_DIR` is used for the find hints for
- `Python_EXECUTABLE`: Unnecessary because we provide `Python_EXECUTABLE` as a cache variable and that has precedence
- `python-config`: should point to the target's variable. Generally this is a shell script which would be executable during cross-compilation (windows case unknown)
- `Python_LIBRARY`: should point to the target's library which would be on the

`crossenv` uses a fake environment so that `sys.base_*_prefix` variables point to the target's system. Presumably PEP720 would work the same.

Signed-off-by: Cristian Le <[email protected]>

Author: LecrisUT

src/scikit_build_core/builder/builder.py

@@ -240,7 +240,7 @@ def configure(
         # Modern Find Python
         for prefix in ("Python", "Python3"):
             cache_config[f"{prefix}_EXECUTABLE"] = sys.executable
-            cache_config[f"{prefix}_ROOT_DIR"] = sys.prefix
+            cache_config[f"{prefix}_ROOT_DIR"] = sys.base_exec_prefix
             cache_config[f"{prefix}_INCLUDE_DIR"] = python_include_dir
             cache_config[f"{prefix}_FIND_REGISTRY"] = "NEVER"
             # FindPython may break if this is set - only useful on Windows