Put PATH in $_ENV after we adjust it so ExecFuture/etc pick it up

Summary: The change to setEnv() means this may not propagate correctly in
some cases. See IRC.

Auditors: btrahan

src/infrastructure/env/PhabricatorEnv.php

@@ -108,6 +108,10 @@ final class PhabricatorEnv {
     }
     putenv('PATH='.$env_path);
 
+    // Write this back into $_ENV, too, so ExecFuture picks it up when creating
+    // subprocess environments.
+    $_ENV['PATH'] = $env_path;
+
     PhabricatorEventEngine::initialize();
 
     $translation = PhabricatorEnv::newObjectFromConfig('translation.provider');