Use deflate for intermediate

ci/build-tarballs.sh

@@ -78,9 +78,7 @@ fi
 mktarball() {
   dir=$1
   if [ "$fmt" = "tar" ]; then
-    # this is a bit wonky, but the goal is to use `xz` with threaded compression
-    # to ideally get better performance with the `-T0` flag.
-    tar -cvf - -C tmp $dir | xz -9 -T0 > dist/$dir.tar.xz
+    tar -czvf dist/$dir.tar.gz -C tmp $dir
   else
     # Note that this runs on Windows, and it looks like GitHub Actions doesn't
     # have a `zip` tool there, so we use something else