nightly builds: Make generated time based version number more sane

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.