Add option to use existing cache but don't save/update cache

[Noisyfox]

Our project generates huge flatpak caches over 4GB, which quickly run out of github's cache limit (10GB).

It will be greate to have an option that enables the cache but don't save new caches, so we can do things like only master build publishes new caches, and all PR builds reuses cache from master build. PR builds tends to not having too much changes from the master so reusing the same cache could still speed up the build process.

Due to how github cache work, the cache generated from PR builds will never be reused by other branches, so there is no need to keep them for us. By disabling cache saving for PR branches we could prevent accidentaly ran out of cache storage and get the master cache purged by github, then every PR builds generates their own caches then keeps geting purged by github, and ever build became terriably slow.