nightly builds: Make generated time based version number more sane
Previously we used:
* date (without time) for the upstream version. This prevented more
than one build per day
* unix timestamp in the package release with prefix "nightlybuild"
Now we use:
* date and time (UTC) for the upstream version. It allows us to make
at most one automated build per minute. That should suffice. :-)
* Hardcoded package release number with shorter prefix (nightly). It
isn't important anymore.
Overall the version number is much shorter and more human readable
than before.