Bug 1894412 - When reading /etc/os-release, fall back to BUILD_ID when VERSION_ID is not set.  a=dmeehan

Arch Linux doesn't come with a VERSION_ID at all, but it has a BUILD_ID.
Semantically, falling back to BUILD_ID when VERSION_ID is not present
would seem fair play:

Per https://www.freedesktop.org/software/systemd/man/latest/os-release.html:

BUILD_ID=
    A string uniquely identifying the system image originally used as the
    installation base. In most cases, VERSION_ID or IMAGE_ID+IMAGE_VERSION
    are updated when the entire system image is replaced during an update.
    BUILD_ID may be used in distributions where the original installation
    image version is important: VERSION_ID would change during incremental
    system updates, but BUILD_ID would not. This field is optional.

Original Revision: https://phabricator.services.mozilla.com/D209089

Differential Revision: https://phabricator.services.mozilla.com/D209192