intel: use a flag instead of setting PYTHONPATH

intel: use a flag instead of setting PYTHONPATH

Meson doesn't allow setting environment variables for custom targets, so
we either need to not pass this as an environment variable or use a
shell script to wrap the invocation. The chosen solution has the
advantage of working for both autotools and meson.

v2: - put rules back in top scope (Ken)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>