Utilize flexibility of Site#in_dest_dir

[ashmaroli]

• This is a 🔨 code refactoring change.

Summary

Site#in_dest_dir is a flexible method that joins multiple arguments into an absolute path:

jekyll/lib/jekyll/site.rb

Lines 411 to 415 in f8c7208

	def in_dest_dir(*paths)
	paths.reduce(dest) do |base, path|
	Jekyll.sanitized_path(base, path)
	end
	end

Therefore, it is unnecessary to generate paths beforehand.

[DirtyF]

Looks good,
Jekyll website builds in 4.4s instead of 5.6s on master.

[ashmaroli]

Thanks.
@jekyllbot: merge +fix