Enable configurability of the Drush cache directory. (#3597)

Author: weitzman

examples/example.drush.yml

@@ -71,6 +71,9 @@ drush:
       - '${env.home}/.drush/sites'
       - /etc/drush/sites
 
+    # Specify a folder where Drush should store its file based caches. If unspecified, defaults to $HOME/.drush.
+    cache-directory: /tmp/.drush
+
 # This section is for setting global options.
 options:
   # Specify the base_url that should be used when generating links.

src/Config/DrushConfig.php

@@ -39,6 +39,7 @@ public function tmp()
     public function cache()
     {
         $candidates = [
+            $this->get('drush.paths.cache-directory'),
             Path::join($this->home(), '.drush/cache'),
             Path::join($this->tmp(), 'drush-' . $this->user() . '/cache'),
         ];