Set a version number containing the build date

On Endless OS we set a version number in two places:

• In the commit metadata, using the key OSTREE_COMMIT_META_KEY_VERSION. This is shown in the output of ostree admin status
• In /usr/lib/os-release, as the fields VERSION & BUILD_ID. If you go and read the os-release spec, this is actually wrong, and IMAGE_VERSION should be used instead. But we've done it this way for nearly a decade, and BUILD_ID has the nice property that it is actually shown in GNOME Settings' About page already. 😄

The commit metadata version takes one of 3 forms:

• Nightly builds: 5.0.2~230301-230009 (that's a yymmdd-hhmmss UTC timestamp)
• Betas: 5.0.2~beta1
• Stable releases: 5.0.2

In this example, VERSION is always 5.0.2, and BUILD_ID is always the build timestamp. IMO it would be useful for GNOME OS to set the same/similar keys, to identify a specific build. It's easier to talk about a specific build of the OS if it is shown in Settings' About page and in ostree admin status.