Fix sassc: copy libsass into ext dir when Ruby >= 4.1

[eightbitraptor]

RubyGems no longer copies compiled extensions into the gem's lib/ tree (ruby/rubygems@1614b036f7), breaking sassc which uses FFI with hardcoded paths.

This commit copies libsass from the extensions directory into ext where sassc's native.rb expects it. Sassc is deprecated and publically archived anyway so there seems little point updating this upstream.

I chose not to use
Gem.configuration.install_extension_in_lib. Because it would set the behaviour for all gems in the bundle, and we'd still have to copy manually anyway, becuase the bundle isn't rebuilt between runs.

[k0kubun]

Does the shipit benchmark compile sass files? If it's loaded in Bundler.require and the loading itself is the only problem, can we add gem "sassc", require: false or something to skip loading the gem?

[eightbitraptor]

I don't think that's going to be possible because the dependency chain that ends up requiring sassc is going through shipit-engine -> sass-rails -> sassc-rails -> sassc.

Shipit::Engine is mounted in the routes as part of the benchmark, so as soon as we require it we're going to end up pulling in sassc.

[k0kubun]

I think we should ideally check the rubygems version instead. It's possible to update rubygems for an older Ruby.

[hsbt]

Suggested change

	if RUBY_VERSION >= "4.1"
	unless Gem.install_extension_in_lib

We may use Gem.install_extension_in_lib instead of version condition.