Fix parallel xz compression

mmdebstrap

@@ -161,7 +161,7 @@ sub get_tar_compressor($) {
     } elsif ($filename =~ /\.lz4$/) {
 	return 'lz4';
     } elsif ($filename =~ /\.(xz|txz)$/) {
-	return 'xz';
+	return ('xz', '--threads=0');
     } elsif ($filename =~ /\.zst$/) {
 	return 'zstd';
     }
@@ -2443,11 +2443,6 @@ sub main() {
     my $exitstatus = 0;
     my @taropts = ('--sort=name', "--mtime=\@$mtime", '--clamp-mtime', '--numeric-owner', '--one-file-system', '-c', '--exclude=./dev');
 
-    # Use parallel xz compression
-    if (defined $tar_compressor and $tar_compressor eq 'xz') {
-	push @taropts, '-Ixz --threads=0'
-    }
-
     # disable signals so that we can fork and change behaviour of the signal
     # handler in the parent and child without getting interrupted
     my $sigset = POSIX::SigSet->new(SIGINT, SIGHUP, SIGPIPE, SIGTERM);