Support environment variable to skip Yarn install (#125)

Similar to rails/jsbundling-rails#150

Author: tmaier

lib/tasks/cssbundling/build.rake

@@ -1,10 +1,18 @@
 namespace :css do
+  desc "Install JavaScript dependencies"
+  task :install do
+    unless system "yarn install"
+      raise "cssbundling-rails: Command install failed, ensure yarn is installed"
+    end
+  end
+
   desc "Build your CSS bundle"
-  task :build do
-    unless system "yarn install && yarn build:css"
+  build_task = task :build do
+    unless system "yarn build:css"
       raise "cssbundling-rails: Command css:build failed, ensure yarn is installed and `yarn build:css` runs without errors or use SKIP_CSS_BUILD env variable"
     end
   end
+  build_task.prereqs << :install unless ENV["SKIP_YARN_INSTALL"]
 end
 
 unless ENV["SKIP_CSS_BUILD"]