improve tar packaging

The generated tar archives contain the uid/gid of the builder,
if unpacked as root on another system those will denote nothing
or an unexpected user/group.

The expectation would be uid/gid = 0.

Use a simple enough tar format as well.

The Mac version of tar (bsdtar?) does not support those flags,
for CI the separate gnu-tar is installed.

release-macos.sh

@@ -33,7 +33,7 @@ rm -rf $DEST
 time CLEAN=1 SYNC=1 MACOS_REDIST=1 ./build-all.sh $DEST
 dir=$(pwd)
 cd $HOME
-tar -Jcvf $dir/$RELNAME.tar.xz $RELNAME
+gtar -Jcvf $dir/$RELNAME.tar.xz --format=ustar --numeric-owner --owner=0 --group=0 $RELNAME
 rm -rf $RELNAME
 cd $dir
 ls -lh $RELNAME.tar.xz