FIX: Do not @import .css assets for plugins (#32014)

b1924c35 switched our compiler to use `@import` internally for scss
entrypoints. This logic also applied to `.css` files, but unfortunately
sass doesn't do anything with `@import` of CSS files, so they'll be left
intact all the way to the browser. Continue using the old concatenation
approach for them in the compiler.

Followup to b1924c352487ab2c85ae50af45c5b3e098589014

lib/stylesheet/compiler.rb

@@ -24,7 +24,11 @@ module Stylesheet
 
         plugin_assets.each do |src|
           options[:load_paths] << File.expand_path(File.dirname(src))
-          file += "@import \"#{src}\";\n"
+          if src.end_with?(".scss")
+            file += "@import \"#{src}\";\n"
+          else
+            file += File.read(src)
+          end
         end
       else # Core asset
         file += "@import \"#{asset}\";\n"