[ci] Remove Pythia8 from mac26 builds

Author: dpiparo

.github/workflows/root-ci-config/buildconfig/mac26.txt

@@ -26,6 +26,7 @@ builtin_zstd=ON
 check_connection=ON
 cocoa=ON
 minuit2_omp=OFF
+pythia8=OFF
 test_distrdf_dask=ON
 test_distrdf_pyspark=ON
 x11=OFF