test: 1.4.20 get xapian working under github actions and python 3.13

issue2551338 xapian doesn't build in CI for 3.13 python

binary search for working version of xapain tarball. try 1.4.20

1.4.22 failed with:

xapian_wrap.cc:65031:116: error: ‘FLAG_NO_POSITIONS’ is not a member
of ‘Xapian::QueryParser’

[skip travis]

.github/workflows/build-xapian.yml

@@ -82,7 +82,7 @@ jobs:
           # Sphinx required to build the xapian python bindings. Use 1.8.5 on
           # older python and newest on newer.
           if [[ $PYTHON_VERSION == '3.'* ]] ; then pip install sphinx; fi
-          XAPIAN_VER="1.4.22"; echo $XAPIAN_VER;
+          XAPIAN_VER="1.4.20"; echo $XAPIAN_VER;
           cd /tmp
           curl -s -O https://oligarchy.co.uk/xapian/$XAPIAN_VER/xapian-bindings-$XAPIAN_VER.tar.xz
           tar -Jxvf xapian-bindings-$XAPIAN_VER.tar.xz