yampl Build Update, master branch (2021.01.04.)

Updated the yampl build to use a tarball (put in place with ATLINFR-3910) instead of downloading it from GitHub directly.

This is meant to solve the incremental build errors that started showing up recently. Probably because of some updates in the behaviour of GitHub itself. Though it's probable that !767 (merged) also had something with the breaking of the CI jobs... 🤔

Failures of the incremental builds have been described in ATLINFR-3911 and athena!39255 (comment 4055139).