cache-root is required when cache is disabled

[seandilda]

rubocop will fail if it can't write to the cache-root directory even if you specify --cache=false

---

Expected behavior

When caching is disabled, rubocop should not crash if it can't write to the cache root.

Actual behavior

rubocop crashes with the error /usr/local/lib/ruby/3.1.0/fileutils.rb:243:in mkdir': Permission denied @ dir_s_mkdir - /.cache (Errno::EACCES)`

Steps to reproduce the problem

$ docker run --rm -it --user 1000 ruby:3.1 bash -c 'gem install rubocop && rubocop --debug --cache=false'          
Fetching rubocop-ast-1.27.0.gem
Fetching regexp_parser-2.7.0.gem
Fetching unicode-display_width-2.4.2.gem
Fetching rainbow-3.1.1.gem
Fetching ruby-progressbar-1.13.0.gem
Fetching ast-2.4.2.gem
Fetching parser-3.2.1.1.gem
Fetching parallel-1.22.1.gem
Fetching rubocop-1.48.1.gem
Successfully installed unicode-display_width-2.4.2
Successfully installed ruby-progressbar-1.13.0
Successfully installed ast-2.4.2
Successfully installed parser-3.2.1.1
Successfully installed rubocop-ast-1.27.0
Successfully installed regexp_parser-2.7.0
Successfully installed rainbow-3.1.1
Successfully installed parallel-1.22.1
Successfully installed rubocop-1.48.1
9 gems installed
/usr/local/lib/ruby/3.1.0/fileutils.rb:243:in `mkdir': Permission denied @ dir_s_mkdir - /.cache (Errno::EACCES)
	from /usr/local/lib/ruby/3.1.0/fileutils.rb:243:in `fu_mkdir'
	from /usr/local/lib/ruby/3.1.0/fileutils.rb:221:in `block (2 levels) in mkdir_p'
	from /usr/local/lib/ruby/3.1.0/fileutils.rb:219:in `reverse_each'
	from /usr/local/lib/ruby/3.1.0/fileutils.rb:219:in `block in mkdir_p'
	from /usr/local/lib/ruby/3.1.0/fileutils.rb:211:in `each'
	from /usr/local/lib/ruby/3.1.0/fileutils.rb:211:in `mkdir_p'
	from /usr/local/lib/ruby/3.1.0/pathname.rb:585:in `mkpath'
	from /usr/local/bundle/gems/rubocop-1.48.1/lib/rubocop/server/cache.rb:46:in `block in dir'
	from <internal:kernel>:90:in `tap'
	from /usr/local/bundle/gems/rubocop-1.48.1/lib/rubocop/server/cache.rb:45:in `dir'
	from /usr/local/bundle/gems/rubocop-1.48.1/lib/rubocop/server/cache.rb:98:in `pid_path'
	from /usr/local/bundle/gems/rubocop-1.48.1/lib/rubocop/server/cache.rb:118:in `pid_running?'
	from /usr/local/bundle/gems/rubocop-1.48.1/lib/rubocop/server.rb:36:in `running?'
	from /usr/local/bundle/gems/rubocop-1.48.1/exe/rubocop:11:in `<top (required)>'
	from /usr/local/bundle/bin/rubocop:25:in `load'
	from /usr/local/bundle/bin/rubocop:25:in `<main>'