Skip dartsass:build if environment variable SKIP_CSS_BUILD is set.

[natsuokawai]

Hello,

dartsass-rails gem currently runs dartsass:build automatically during assets:precompile.
This behavior is enforced by the code here:

dartsass-rails/lib/tasks/build.rake

Line 35 in d227ae6

Rake::Task["assets:precompile"].enhance(["dartsass:build"])

However, in some scenarios, it's beneficial to run dartsass:build and assets:precompile separately.

For example, the cssbundling-rails gem offers a way to skip its build process during assets:precompile by setting an environment variable SKIP_CSS_BUILD, as introduced by rails/cssbundling-rails#99.

To align dartsass-rails with this approach, I suggest a similar feature.