Update actions/cache to v4 to suppress warnings about old node.js versions
Allow users to set PUB_CACHE before invoking the action to control where the .pub-cache directory lives
Cache the .pub-cache directory separately from the flutter installation, since the former will change often and the latter will change rarely -- this reduces the amount of cache churn / space used up in the cache until the data expires
PUB_CACHE
before invoking the action to control where the .pub-cache directory lives