make-options-docs: don't sort the options XML file

we need the file itself as a dependency for the docbook build, but we don't need
it to be properly sorted at the nix level. push the sort out to a python script
instead to save eval time. on the machine used to write this `nix-instantiate
<nixos/nixos> -A system` went down from 7.1s to 5.4s and GC heap size decreased
by 50MB (or 70MB max RSS).

nixos/lib/make-options-doc/sortXML.py

@@ -0,0 +1,28 @@
+import xml.etree.ElementTree as ET
+import sys
+
+tree = ET.parse(sys.argv[1])
+# the xml tree is of the form
+# <expr><list> {all options, each an attrs} </list></expr>
+options = list(tree.getroot().find('list'))
+
+def sortKey(opt):
+    def order(s):
+        if s.startswith("enable"):
+            return 0
+        if s.startswith("package"):
+            return 1
+        return 2
+
+    return [
+        (order(p.attrib['value']), p.attrib['value'])
+        for p in opt.findall('attr[@name="loc"]/list/string')
+    ]
+
+# always ensure that the sort order matches the order used in the nix expression!
+options.sort(key=sortKey)
+
+doc = ET.Element("expr")
+newOptions = ET.SubElement(doc, "list")
+newOptions.extend(options)
+ET.ElementTree(doc).write(sys.argv[2], encoding='utf-8')