roachtest: print cockroach random seed to test.log

This change prints the cockroach random seed used by metamorphic
builds to test.log for ease of debugging. Before this seed was found
only in cockroach.log.

Release note: none

pkg/cmd/roachtest/cluster.go

@@ -1936,6 +1936,7 @@ func (c *clusterImpl) StartE(
 	// Set the same seed on every node, to be used by builds with
 	// runtime assertions enabled.
 	setUnlessExists("COCKROACH_RANDOM_SEED", c.cockroachRandomSeed())
+	l.Printf("ROACHTEST COCKROACH_RANDOM_SEED: %d", c.cockroachRandomSeed())
 
 	// Needed for backward-compat on crdb_internal.ranges{_no_leases}.
 	// Remove in v23.2.