In order to build aarch64 packages it relies on the machine executors
feature based on QEMU. Since the same scripts were needed to build
packages for different architectures they were extracted in standalone
shell scripts.

Alpine Linux packages are now built in addition to RPM and Deb packages
but considering how long emulation takes for aarch64 packages, packaging
was moved to a nightly workflow.

Some changes happened in the pkg-varnish-cache repository to accomodate
the CircleCI reorganization, mainly around dependency management.

More information in the CircleCI README file.

Closes #3313
Reviewed-by: Guillaume Quintard <guillaume@varnish-software.com>
Signed-off-by: Dridi Boukelmoune <dridi.boukelmoune@gmail.com>