chore(3p/lisp): use nixpkgs sources for s-xml

this one was a little more difficult because it needs a patch, there's
something wonky with the definition order

fwiw, the upstream cvs repository ... server errors.

Change-Id: I2d99359edec36b578389f1be1fcf077743c29c4e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4342
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

third_party/lisp/s-xml/0001-fix-definition-order-in-xml.lisp.patch

@@ -0,0 +1,26 @@
+From 789dc38399f4039b114de28384c149721d66b030 Mon Sep 17 00:00:00 2001
+From: Vincent Ambo <mail@tazj.in>
+Date: Thu, 16 Dec 2021 00:48:04 +0300
+Subject: [PATCH] fix definition order in xml.lisp
+
+---
+ src/xml.lisp | 3 +++
+ 1 file changed, 3 insertions(+)
+
+diff --git a/src/xml.lisp b/src/xml.lisp
+index 39c9b63..3232491 100644
+--- a/src/xml.lisp
++++ b/src/xml.lisp
+@@ -19,6 +19,9 @@
+ 
+ ;;; error reporting
+ 
++(defvar *ignore-namespaces* nil
++  "When t, namespaces are ignored like in the old version of S-XML")
++
+ (define-condition xml-parser-error (error)
+   ((message :initarg :message :reader xml-parser-error-message)
+    (args :initarg :args :reader xml-parser-error-args)
+-- 
+2.34.0
+