The `rocksdb` version shipped with distributions is typically old and
therefore often lacks features we use - it also doesn't match the one
assumed by nim-rocksdb leading to ABI mismatch risks.
Instead of depending on the system rocksdb, we'll now use the rocksdb
version assumed by nim-rocksdb and locked in its vendor folder by always
building it together with nimbus.
This avoids the problem of unknown rocksdb versions at a (small) cost to
build time.
CI caching and full windows support for building from source [remains
TODO](https://github.com/status-im/nim-rocksdb/issues/44).
95a4adc1e8