roachtest/workload: use uint seeds

Roachtests typically use a in64 seed set[ here ](https://github.com/msbutler/cockroach/blob/butler-deflake-br-roundtrip/pkg/util/randutil/rand.go#L73), but workload can only [take](https://github.com/msbutler/cockroach/blob/butler-deflake-br-roundtrip/pkg/workload/random.go#L64) a uint seed. It would be nice if we used positive seeds everywhere in testing. I realize a simple workaround is to generate a random positive int to pass as a seed to the workload command, like in this [pr](https://github.com/cockroachdb/cockroach/pull/154154).

Jira issue: CRDB-54764

Start with pkg/util/randutil/rand.go around the seed definition and pkg/workload/random.go around workload seed handling. Compare the workaround in PR 154154, then trace roachtest and workload seed call sites to make seed types consistent; done means testing uses positive seeds without requiring ad hoc conversion.

Written by the indexing model from the issue text.