timezone setup unreliable

So here's what I observed:

% vagrant init -m --box-version 0.5 fdroid/basebox-stretch64
% vagrant up --provider libvirt
% vagrant ssh -c 'cat /etc/timezone'
Etc/UTC
% vagrant ssh -c 'date'
Fri 21 Sep 18:43:11 UTC 2018

% vagrant init -m --box-version 0.5 fdroid/basebox-stretch64
% vagrant up --provider virtualbox
% vagrant ssh -c 'cat /etc/timezone'
Etc/UTC
% vagrant ssh -c 'date'
Fri 21 Sep 20:45:24 CEST 2018

Timezone should be the same on both. (CEST is the timezone of the host system where I'm doing tests.)

also see: !18 (merged) and !17 (merged)