force Vagrant to use the "new" API URL

It seems Vagrant has introduced a new API and their proprietary client
does use that by default now, so they turned off the old one (or the
redirect, I really have no idea what was there last week).

Just setting the new URL (which I found out by using `vagrant --debug`)
as the server URL works for now, so let's do that, and at least unblock
nightlies.

roles/forklift/tasks/up.yml

@@ -29,6 +29,8 @@
       command: "vagrant up --no-parallel {{ forklift_boxes.keys()|join(' ') }}"
       args:
         chdir: "{{ forklift_directory }}"
+      environment:
+        VAGRANT_SERVER_URL: https://vagrantcloud.com/api/v2/vagrant
   always:
     - name: Remove lock file
       ansible.builtin.file: