cygwin CI: install python312-lxml instead of python39-lxml

The Cygwin meson runtime resolves to /usr/bin/python3.12.exe (python312 is
pulled in transitively as a meson dependency), while the workflow only
installed python39-lxml. hw/xwin/glx/meson.build therefore failed with
"python3 lxml module not found" on every PR.

Install the lxml module for the Python interpreter meson actually uses.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

.github/workflows/build-xserver.yml

@@ -418,7 +418,7 @@ jobs:
                    libxcb-render-devel, libxcb-render-util-devel,
                    libxcb-shape-devel, libxcb-util-devel, libxcb-xkb-devel,
                    libxcvt-devel, libxkbfile-devel, font-util,
-                   khronos-opengl-registry, python39-lxml, xkbcomp-devel,
+                   khronos-opengl-registry, python312-lxml, xkbcomp-devel,
                    xkeyboard-config, git
 
             - name: Cygwin ccache cache