In v2, zstd is required for a cache restore.

That means that all self-hosted runners that don't have zstd installed will simply give a cache miss on trying to restore. No explanation.

There are a number of issues here:

1. Since zstd is required, fail on cache restore if zstd is not found.
2. Since gnu tar is required (apparently) to use zstd, fail if gnu tar is not found.
3. Add the requirement of zstd to the README
4. Don't output that it's a cache miss when it's not at all a cache miss but rather an inability to untar it.