Cleanup irbrc generator cache always at teardown

[tompng]

Fix this Error loading RC file message shown while running test_init.rb

$ rake test TEST=test/irb/test_init.rb 
Error loading RC file '/var/folders/zj/fs39x31j7s9b2lp7jydtmxgr0000gq/T/irbrc20240612-98621-vgoozc':
/Users/tomoya.ishida/github/ruby/irb/lib/irb/init.rb:400:in `load': cannot load such file -- /var/folders/zj/fs39x31j7s9b2lp7jydtmxgr0000gq/T/irbrc20240612-98621-vgoozc (LoadError)
	from /Users/tomoya.ishida/github/ruby/irb/lib/irb/init.rb:400:in `block in run_config'
	from /Users/tomoya.ishida/github/ruby/irb/lib/irb/init.rb:399:in `each'
	from /Users/tomoya.ishida/github/ruby/irb/lib/irb/init.rb:399:in `run_config'
	from /Users/tomoya.ishida/github/ruby/irb/lib/irb/init.rb:54:in `setup'
	from /Users/tomoya.ishida/github/ruby/irb/test/irb/test_init.rb:171:in `test_completor_environment_variable'
	from /Users/tomoya.ishida/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/test-unit-3.6.1/lib/test/unit/testcase.rb:871:in `run_test'
	...

[ima1zumi]

LGTM