Fix python site-packages dir

[NTULINUX]

2.9 backport

[grandixximo]

CI failure on package-arch (bullseye) is real. Bullseye has python3.9: no deb_system (added in 3.10), no rpm_prefix (Fedora-only), and sysconfig.get_default_scheme() does not exist before 3.10, so the new line raises AttributeError and SITEPY comes out empty. Verified in a bullseye container. Empty SITEPY drops the python tree into $(DESTDIR) root, usr/lib/python3 in debian/linuxcnc.install.in matches nothing, package build fails. Master never hit this; it no longer builds bullseye.

get_default_scheme() is the wrong fallback anyway: it yields site-packages, but Debian pythons without deb_system only import from dist-packages. Suggested fix, works on 3.9+:

SITEPY=`$PYTHON -c "import sysconfig; s=sysconfig.get_scheme_names(); print(next((sysconfig.get_path('platlib', m) for m in ('deb_system', 'rpm_prefix') if m in s), '/usr/lib/python3/dist-packages'))"`