container/builder: configure plugin via cfg file

Matches the config of the containers.

run-builder.sh

@@ -22,6 +22,7 @@ fi
 ${CONTAINER_RUNTIME} run --rm -i -t --name org.osbuild.koji.builder --network org.osbuild.koji \
   -v "${SHARE_DIR}:/share:z" \
   -v "${DATA_DIR}:/mnt:z" \
+  -v "${PWD}/container/builder/osbuild-koji.conf:/etc/osbuild-composer/koji.conf:z" \
   --hostname org.osbuild.koji.kojid \
   --add-host=composer:$(ip route show dev cni-podman0 | cut -d\  -f7) \
   koji.builder