[Downloader] Run prepare_command even when skipping the cache

CHANGELOG.md

@@ -13,6 +13,13 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
   [Samuel Giddins](https://github.com/segiddins)
   [#2443](https://github.com/CocoaPods/CocoaPods/issues/2443)
 
+##### Bug Fixes
+
+* Ensure that the `prepare_command` is run even when skipping the download
+  cache.  
+  [Samuel Giddins](https://github.com/segiddins)
+  [#3674](https://github.com/CocoaPods/CocoaPods/issues/3674)
+
 
 ## 0.38.0.beta.1
 

lib/cocoapods/downloader.rb

@@ -33,7 +33,9 @@ module Pod
         cache = Cache.new(cache_path)
         result = cache.download_pod(request)
       else
+        require 'cocoapods/installer/pod_source_preparer'
         result, _ = download_request(request, target)
+        Installer::PodSourcePreparer.new(result.spec, result.location).prepare!
       end
 
       if target && result.location && target != result.location