acme: Use chown -R for challenges directory. Fixes #24529.

Commit 75f131da02c00027b9a8240fb74d117cb0f9d9cf added
`chown 'nginx:nginx' '/var/lib/acme'` to the pre-start script,
but since it doesn't use `chown -R`, it is possible that there
are older existing subdirs (like `acme-challenge`)
that are owned to `root` from before that commit went it.

nixos/modules/security/acme.nix

@@ -178,7 +178,7 @@ in
                   path = [ pkgs.simp_le ];
                   preStart = ''
                     mkdir -p '${cfg.directory}'
-                    chown '${data.user}:${data.group}' '${cfg.directory}'
+                    chown -R '${data.user}:${data.group}' '${cfg.directory}'
                     if [ ! -d '${cpath}' ]; then
                       mkdir '${cpath}'
                     fi