And you cannot clear the cache, hehe, quite limited right now for sure

I didn't look it further initially because the documentation was linking to an issue and saying that it wasn't ready but they actually merged a change which basically restores a cache with a partial match on the cache key. This means that the cache being immutable is fine, what you do is to give a static part to the key, like

ccache

and then add a variable part like the commit id. Then you can specify a restore key, meaning a prefix that can be used to retrieve a list of caches, and the latest will be used. So you you give it

ccache

. TLDR: I've tested it better, it works, the Linux Release build goes down to ~10m. There's to say though that the task is still considered in beta and not for production.. so even for a security point of view I would wait