[madgraph2021 v0.6, MRs 646+647] merge qa-build-mg5amc-madgraph4gpu commit 90148225 from 4377015

Changed the title to mention "[madgraph2021 v0.6, MRs 646+647]"

This merges into master the changes for mg2021 and infrastructure in !646 (merged) (where madgraph2021 v0.6 was built) and !647 (merged).