python310Packages.seaborn: fix sandboxless build (#216254)

pkgs/development/python-modules/seaborn/default.nix

···
     "TestKDEPlotUnivariate"
     "test_with_rug"
     "test_bivariate_kde_norm"
+
+    # requires internet connection
+    "test_load_dataset_string_error"
   ] ++ lib.optionals (!stdenv.hostPlatform.isx86) [
     # overly strict float tolerances
     "TestDendrogram"
   ];
+
+  # All platforms should use Agg. Let's set it explicitly to avoid probing GUI
+  # backends (leads to crashes on macOS).
+  MPLBACKEND="Agg";
 
   pythonImportsCheck = [
     "seaborn"