ci: Introduce $(CI_BUILD_SCRIPT)

ci: Introduce $(CI_BUILD_SCRIPT)

Instead of hardcoding build instructions into the Makefile,
move them to a separate script that's mounted into the
container.

This gives us a couple of advantages: we no longer have to
deal with the awkward quoting required when embedding shell
code in a Makefile, and we also provide the users with a way
to override the default build instructions with their own.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>