Consider how to create stable and unique cache keys

[hinshun]

A common pattern among all build workflows is using cache mounts. But in order to use it, we need to provide a cache key. If we're using the same HLB for multiple projects, or multiple users using the same BuildKit we need their cache keys to not collide.

For example, we can do:

run "foobar" with option {
    mount fs { scratch; } "/cache" with option {
        cache string { hash string { getEnv "CWD"; }; }
    }
}