This adds a machine-independent job "build-version" which populates the
RELEASE_VERSION and RELEASE_NAME variables, so that following jobs can
use these without depending on the various build jobs.
The variables can be set from the trigger job in a project's
`.gitlab-ci.yml` file. They are eval'ed before saving them to
version.env, so we can use deferred variable expansion or even command
execution to construct their values. This mechanism is already used for
the Flash-N-Go System variables.