Add gurobi 11

Maybe to merge before the externals which would install only gurobi 11, I suppose

ci/update_squashfs.sh

@@ -22,6 +22,7 @@ jq -Mr '
   | .[]
 ' ${STACK_CONFIG} > excludes.list
 echo buildcache >> excludes.list
+echo external >> excludes.list
 
 cd /squashfs-cache