[ci] Use non-interactive matplotlib backend on Windows.

Fixes FAILED tests/test_plot.py::TestPlotting::test_save_animation - _tkinter.TclError: invalid command name "tcl_findLibrary"

See e.g. https://github.com/materialsproject/pymatgen/pull/4168

.gitlab-ci.yml

@@ -152,6 +152,8 @@ tests (win - pip) [R]:
   rules:
     - *regression-rules
   needs: []
+  variables:
+    MPLBACKEND: Agg  # non-interactive backend for matplotlib
   before_script:
     - python -m venv .venv
     - .venv\Scripts\Activate.ps1