[rubygems/rubygems] Report Github Actions CI provider within user agent string

See
https://docs.github.com/es/actions/learn-github-actions/environment-variables#default-environment-variables.

So that we can show stats about it at https://ecosystem.rubytogether.org

https://github.com/rubygems/rubygems/commit/48d6dc68f8

lib/bundler/fetcher.rb

@@ -226,6 +226,7 @@ module Bundler
         "GO_SERVER_URL" => "go",
         "SNAP_CI" => "snap",
         "GITLAB_CI" => "gitlab",
+        "GITHUB_ACTIONS" => "github",
         "CI_NAME" => ENV["CI_NAME"],
         "CI" => "ci",
       }