feat: autopush to S3 buildcache

This PR enables autopush for S3 repositories, ensuring the built packages are written to S3 as soon as built (in fact, as part of the package build process). This makes sure we don't lose builds.

However, the main reason is to use spack functionality instead of makefile functionality for this and keep the definition of autopush together with the buildcache definition instead of entirely elsewhere.

(Note: At this point only S3 buildcaches support autopush since for OCI buildcaches the dependency stack needs to be available.)