When building a config stack, stop SiteSource objects from poisoning the cache

Summary:
Ref T13168. I'm not sure how this worked before, but I ran into this issue on my new laptop.

SiteSource accesses `PhabrictatorEnv::getEnvConfig('phabricator.base-uri')` when local, which may poison the cache and lock the value since we don't later discard the cache.

Specifically, when I access `http://locala.phacility.com`, I was getting an error like "You made a request for locala.phacility.com, but no configured site can serve this request.". This was because the base-uri was being incorrectly frozen as "local.phacility.com". The expectation is that it will match, so the standard PlatformSite will serve the request.

Test Plan:
  - Before change: "no configured site" error.
  - After change: local instance works properly.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13168

Differential Revision: https://secure.phabricator.com/D19526

src/infrastructure/env/PhabricatorEnv.php

@@ -221,6 +221,10 @@ final class PhabricatorEnv extends Phobject {
 
     foreach ($site_sources as $site_source) {
       $stack->pushSource($site_source);
+
+      // If the site source did anything which reads config, throw it away
+      // to make sure any additional site sources get clean reads.
+      self::dropConfigCache();
     }
 
     $masters = PhabricatorDatabaseRef::getMasterDatabaseRefs();
@@ -259,6 +263,10 @@ final class PhabricatorEnv extends Phobject {
         throw $ex;
       }
     }
+
+    // Drop the config cache one final time to make sure we're getting clean
+    // reads now that we've finished building the stack.
+    self::dropConfigCache();
   }
 
   public static function repairConfig($key, $value) {