fluentd: fix service start up

"rexml" was moved from default to bundled gems in ruby 3.0.0. This causes fluentd service to fail with the error "aws-sdk-core/xml/parser.rb in `set_default_engine': Unable to find a compatible xml library)". This fix provides default xml engine and fixes fluentd startup problem.

pkgs/tools/misc/fluentd/Gemfile.lock

···
       aws-partitions (~> 1, >= 1.651.0)
       aws-sigv4 (~> 1.5)
       jmespath (~> 1, >= 1.6.1)
+      nokogiri (~> 1.0)
     aws-sdk-firehose (1.50.0)
       aws-sdk-core (~> 3, >= 3.165.0)
       aws-sigv4 (~> 1.1)
···
     http_parser.rb (0.8.0)
     jmespath (1.6.2)
     ltsv (0.1.2)
+    mini_portile2 (2.8.2)
     mongo (2.18.2)
       bson (>= 4.14.1, < 5.0.0)
     msgpack (1.6.0)
     multi_json (1.15.0)
     multipart-post (2.2.3)
+    nokogiri (1.15.2)
+      mini_portile2 (2.8.2)
+      racc (1.6.2)
     public_suffix (5.0.1)
+    racc (1.6.2)
     rake (13.0.6)
     ruby-kafka (1.5.0)
       digest-crc

pkgs/tools/misc/fluentd/gemset.nix

···
 {
+  mini_portile2 = {
+    groups = ["default" "development" "test"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "0z7f38iq37h376n9xbl4gajdrnwzq284c9v1py4imw3gri2d5cj6";
+      type = "gem";
+    };
+    version = "2.8.2";
+  };
+  racc = {
+    groups = ["default" "development" "test"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "09jgz6r0f7v84a7jz9an85q8vvmp743dqcsdm3z9c8rqcqv6pljq";
+      type = "gem";
+    };
+    version = "1.6.2";
+  };
+  nokogiri = {
+    dependencies = ["mini_portile2" "racc"];
+    groups = ["default" "development" "test"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "1mr2ibfk874ncv0qbdkynay738w2mfinlkhnbd5lyk5yiw5q1p10";
+      type = "gem";
+    };
+    version = "1.15.2";
+  };
   addressable = {
     dependencies = ["public_suffix"];
     groups = ["default"];
···
     version = "1.58.0";
   };
   aws-sdk-core = {
-    dependencies = ["aws-eventstream" "aws-partitions" "aws-sigv4" "jmespath"];
+    dependencies = ["aws-eventstream" "aws-partitions" "aws-sigv4" "jmespath" "nokogiri"];
     groups = ["default"];
     platforms = [];
     source = {