Prevent environment leaking into initrd-release

Prevent environment leaking into initrd-release

On my system the following initrd-release is generated:
...
VERSION="4 dracut-048 dracut-048"
...

VERSION is not defined in /etc/os-release, so the variable is
concatenated with its previous value:

* "4" comes from the kernel build system since dracut is called from the
  kernel install hook ("4" is a major kernel version);
* first "dracut-048" comes from the "systemd-initrd" module;
* second "dracut-048" comes from the "base" module.