All modules with preStart: Use systemd.tmpfiles.rules

This is NixOS' recommended way to setup service dirs
https://github.com/NixOS/nixpkgs/pull/56265. This commit hands off the
initial data directory creation to systemd.tmpfiles.rules. All other
preStart scripts are left intact to limit this changes' scope.

modules/nix-bitcoin-webindex.nix

@@ -28,9 +28,8 @@ let
   '';
   createWebIndex = pkgs.writeText "make-index.sh" ''
     set -e
-    mkdir -p /var/www/
     cp ${indexFile} /var/www/index.html
-    chown -R nginx /var/www/
+    chown -R nginx:nginx /var/www/
     nodeinfo
     . <(nodeinfo)
     sed -i "s/CLIGHTNING_ID/$CLIGHTNING_ID/g" /var/www/index.html
@@ -48,6 +47,10 @@ in {
   };
 
   config = mkIf cfg.enable {
+    systemd.tmpfiles.rules = [
+      "d /var/www 0755 nginx nginx - -"
+    ];
+
     services.nginx = {
       enable = true;
       virtualHosts."_" = {