Fixes <https://bugs.gnu.org/29782>.
Reported by Gábor Boskovits.
* gnu/packages/patches/docbook-xsl-nonrecursive-string-subst.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/docbook.scm (docbook-xsl)[source](patches): Use it.
[native-inputs]: Add XZ.
[arguments]: Adjust PATH accordingly.
Marius Bakke