Clean up nokogumbo extension dir

This removes 46.3 MB of a native C extension work directory.

changelogs/unreleased/sh-clean-nokogumbo-ext.yml

+---
+title: Clean up nokogumbo extension dir
+merge_request: 4698
+author:
+type: other

config/projects/gitlab.rb

@@ -220,6 +220,7 @@ exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/core'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/ruby/ext'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/ruby/spec'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/third_party'
+exclude 'embedded/lib/ruby/gems/*/gems/nokogumbo-*/ext'
 exclude 'embedded/lib/ruby/gems/*/gems/rbtrace-*/ext/src'
 exclude 'embedded/lib/ruby/gems/*/gems/rbtrace-*/ext/dst'
 exclude 'embedded/lib/ruby/gems/*/gems/*pg_query-*/ext'