* Remove the assets:precompile dependency on yarn:install

Since yarn isn't required for Rails 7, assets:precompile should
not depend on it by default. Instead, tasks for webpacker,
jsbundling-rails, cssbundling-rails, etc should depend on
yarn:install on their own.

* In yarn:install, call `yarn` instead of `bin/yarn`

Change to calling `yarn` directly (via PATH) as `bin/yarn` is no longer
a part of Rails 7.