Log Message:
misc/py-installer: Implement `--executable' option

https://github.com/pypa/installer/issue/257
https://github.com/pypa/installer/pull/258

This wille enable us to install Python scripts with #! lines for
LOCALBASE rather than TOOLBASE (or, from another perspective,
CROSS_LOCALBASE rather than LOCALBASE).  For example, if you're doing
cross-builds in /home/user/pkg for /usr/pkg, the Python executable
running installer will be /home/usr/pkg/bin/python3.12 at build-time
but we need to bake /usr/pkg/bin/python3.12 into the build product.