Thread-safety issues on Rubinius

[glebm]

Code like this:

@cache[key] || @mutex.synchronize { @cache[key] ||= fetch_value }

Is not thread-safe and is flaky (fails CI sometimes) on Rubinius.

Instances of this in the codebase are:

i18n-tasks/lib/i18n/tasks/scanners/files/caching_file_reader.rb

Line 25 in 3582b9a

@cache[absolute_path] || @mutex.synchronize { @cache[absolute_path] ||= super }

i18n-tasks/lib/i18n/tasks/scanners/files/caching_file_finder_provider.rb

Lines 22 to 25 in 3582b9a

	def get(**file_finder_args)
	@cache[file_finder_args] || @mutex.synchronize do
	@cache[file_finder_args] ||= begin
	args = file_finder_args.dup

This one should be OK, as the instance variable is set (to nil) in the constructor (so the ivars keys are not modified):

i18n-tasks/lib/i18n/tasks/scanners/files/caching_file_finder.rb

Line 31 in 3582b9a

@cached_paths || @mutex.synchronize { @cached_paths ||= super }