[madgraph2021 build v0.6] Fixes for madgraph cpu-only runs and for docker caching

This ME for Madgraph "2021" is mainly about:

• BMK-983: fixes the issue with CPU-only runs of the container
• it also includes an upgrade of madgraph 2021 to cuda 11.6 (from 11.4) and gcc 10.2 (from 9.2), using the same madgraph code

It also includes fixes for other infrastructure issues:

• BMK-1010: improve docker layer caching in the CI, by keeping newly created docker layers for at least one week in the CI
• BMK-998: clarify that the correct tag for gpu runners is hep-workload-gpu-docker-builder (not hep-workload-docker-builder-gpu, which was an unintentional bug)
• BMK-1008: fix a comment about a patch that previously had been incorrectly documented as related to BMK-1009

This will build the official container v0.6 of madgraph 2021