When a .tarball-version file containing the version string is present,
it is taken for granted that the source tarball is present.  This
allows to lift the requirement on a Git checkout to use the source
tarball.  This makes it easy and cheap to sync this tarball and
accompanying .tarball-version file across different Jenkins agents to
distribute the packaging jobs, bring the total build time from about 1
h 40 to 40 minutes.

* Makefile (TARBALL_VERSION): New variable.  When defined, do not do
not invoke Git to derive the version string, simply use it.
[RELEASE_TARBALL_FILENAME]: Do not compute prerequisites when
TARBALL_VERSION is defined; assume the tarball is present.
* .gitignore: Ignore .tarball-version.
* Jenkinsfile: (Generate release tarball): Generate a .tarball-version
file, and stash it along the release tarball.  Explicit this should
run on a 'guix' agent.
(Build packages): Forward to any 'linux-builder' agent, clean the
workspace, unstash and extract t...