Investigate using the system `librocksdb` in CI

[grtlr]

@pvdrz found out that we can save the recompilation of librocksdb by setting an environment variable: ROCKSDB_LIB_DIR=/usr/lib/librocksdb.so. Maybe we can use this to speed up CI builds too, by installing the library using apt-get.

[pvdrz]

librocksdb-sys requires a rocksdb version compatible with 6.22.1 and Ubuntu repos only provide 6.11.4

We'd have to figure out if that is enough or if we can download the right version from somewhere else.

Edit: it is not possible to use something different than ubuntu so I'm trying to compile and cache rocksdb here: https://github.com/iotaledger/bee/pull/1023