ci: compress xz -6 to be nice to Ken's build server

scripts/ci/debsrcbuilder.sh

@@ -24,8 +24,8 @@ set_fs_ver "$ver" "$major" "$minor" "$micro" "$rev"
 sleep 2
 cd $ddir
 tar -c --exclude=.git -vf $orig.tar $fname
-echo "Compressing $orig.tar with xz -9e..." >&2
-xz -9e $orig.tar
+echo "Compressing $orig.tar with xz -6..." >&2
+xz -6 $orig.tar
 
 cd $bdir
 (cd debian && ./bootstrap.sh)