gitlab: Add script_variables and define $MAKEFLAGS there

This new template provides a standardized place where environment
variables which are not static, but rather depend on the output of
some shell command, can be defined for later use. This pattern is
already used in libosinfo's GitLab CI integration.

Defining $MAKEFLAGS there means we don't need to call getconf over
and over, and the actual build steps don't end up drowned in the
noise.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>