Add an option to pass extra arguments to cargo

This is e.g. useful for conditional compilation with features

Author: konstin

ruby/lib/helix_runtime/project.rb

@@ -100,6 +100,9 @@ def cargo_build
       if ENV['VERBOSE']
         cargo_args << " --verbose"
       end
+      if ENV['CARGO_EXTRA_ARGS']
+        cargo_args << ENV['CARGO_EXTRA_ARGS']
+      end
       if link_args
         rustc_args << "-C link-args=#{link_args}"
       end