nixos/munin: scripts need to be executable in order to build a wrapper

"Builder called die: Cannot wrap
/nix/store/XXX-munin-available-plugins/plugin.sh because it is not an
executable file"

[Bjørn: Keep DRY, quote "$file".]

nixos/modules/services/monitoring/munin.nix

@@ -26,7 +26,9 @@ let
 
       for file in $out/*; do
         case "$file" in
-            plugin.sh) continue;;
+            */plugin.sh|*/plugins.history)
+              chmod +x "$file"
+              continue;;
         esac
 
         # read magic makers from the file