ci: build.sh: Add a helper function to create the dist tarball

ci: build.sh: Add a helper function to create the dist tarball

This helper function does not correspond to a particular GitLab job, it
just logically separates the necessary step of creating a dist tarball
from the RPM build job that takes over.
One notable change here is the need to update git's file index which
causes issues in local container executions which rely on a shallow
copy of the libvirt repo created as:

    $ git clone --local

Even if all changes have been committed, git often complained
otherwise. Updating the index in a GitLab environment is a NOP.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>