Patch blender BPY API generation command

When blender is called to generate sphinx source files for the
documentation, it appears not to find the scripts directory, perhaps
because the command is intended to be called after installation.  Add
command-line arguments to the call in ebuild to direct blender to the
correct location for the scripts.

This does not appear to have been an issue in older versions, which were
able to find the scripts without the extra arguments.

diff --git a/media-gfx/blender/blender-2.83.4-r1.ebuild b/media-gfx/blender/blender-2.83.4-r1.ebuild
index 0123456..0123456 100644
--- a/media-gfx/blender/blender-2.83.4-r1.ebuild
+++ b/media-gfx/blender/blender-2.83.4-r1.ebuild
@@ -238,7 +238,10 @@ src_compile() {
 
 		cd "${CMAKE_USE_DIR}" || die
 		einfo "Generating (BPY) Blender Python API docs ..."
-		"${BUILD_DIR}"/bin/blender --background --python doc/python_api/sphinx_doc_gen.py -noaudio || die "sphinx failed."
+		"${BUILD_DIR}"/bin/blender --background -noaudio                     \
+		                           --env-system-scripts release/scripts      \
+		                           --python doc/python_api/sphinx_doc_gen.py \
+		|| die "blender failed."
 
 		cd "${CMAKE_USE_DIR}"/doc/python_api || die
 		sphinx-build sphinx-in BPY_API || die "sphinx failed."