This is one of the reasons why, in Go, my team vendors our dependencies. For any service that seeks stability and the ability to deploy any time, removing networked build dependencies is important.
Vendoring is checking in your dependencies with the source. You can def consider that a local cache. The next step up is to run your own proxy server (pretty much a package server / mirror). Next is to use a service like artifacory that does similar.
We should call it a backup. Calling it a cache puts people in mind of size/hit ratio tradeoffs; what's needed is zero tolerance for loss of mission critical code.
Is that like a local cache?