Caching is broken in firmware build actions.

[faldor20]

Currently caching is broken for building firmware in a config repo, i checked my builds, and also that of other public config repos, all never use the cache.

I ran some tests with debug on and is seems to be caused by zstd being missing, unfortunately the version of zstd available in your version of ubuntu isn't new enough for the caching to work properly with it.

##[debug]Checking zstd --quiet --version
##[debug]*** zstd command line interface 64-bits v1.4.4, by Yann Collet ***
##[debug]zstd version: null

If I run it with zstd installed, that version doesn't support the quiet flag and so it doesn't get the version properly and fails. All this seems to be causing the cache version to change every single run, and because the version isn't the same it cannot be reused.

This took quite a bit of sleuthing to dig up, so I hope it is useful to you 😃