vagrant: 2.2.14 -> 2.2.15
ChangeLog: https://github.com/hashicorp/vagrant/blob/v2.2.15/CHANGELOG.md#2215-march-30-2021
Also had to revert a change which improved plugin-errors a bit since it
changed the dependency resolution in a way that broke `vagrant-libvirt`
from `$out/vagrant-plugins/plugins.d`.