[SPARK-39596][INFRA][FOLLOW-UP] Install mvtnorm for SparkR documentation build in CI

[HyukjinKwon]

What changes were proposed in this pull request?

This PR is a followup of #36987 that installs mvtnorm too. It fails with complaining the package is missing:

/__w/spark/spark/docs/_plugins/copy_api_dirs.rb:147:in `<top (required)>': R doc generation failed (RuntimeError)
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/external.rb:60:in `require'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/external.rb:60:in `block in require_with_graceful_fail'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/external.rb:57:in `each'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/external.rb:57:in `require_with_graceful_fail'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/plugin_manager.rb:89:in `block in require_plugin_files'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/plugin_manager.rb:87:in `each'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/plugin_manager.rb:87:in `require_plugin_files'
	from /__w/spark/spark/docs/.local_ruby_bundle/ruby/2.7.0/gems/jekyll-4.2.1/lib/jekyll/plugin_manager.rb:21:in `conscientious_require'

https://github.com/apache/spark/runs/7053043596?check_suite_focus=true

I don't know the root cause yet but we should better make it working first.

Why are the changes needed?

To recover the build.

Does this PR introduce any user-facing change?

No, test-only.

How was this patch tested?

CI in this PR should test it out.