ci: new strategy for dune cache

This MR change the cache strategy for dune. All merge_train / merge_result pipelines are now with policy pull-push, while scheduled pipeline with policy pull.

This is a scheduled pipeline to populate the cache : https://gitlab.com/tezos/tezos/-/pipelines/2210988652

THIS IS A POC.

The rational behind this experiment.

https://hackmd.io/@pietro1234/rJbw0IOGZx

Ignore the comments below. I just recycled an abandoned MR.