roachtest: fix clusterImpl.IsLocal()

Previously, c.IsLocal() would always return false because local clusters
have a time based suffix. This patch changes c.IsLocal() to return true if the
cluster name is prefixed with "local".

Epic: none

Release note: none

pkg/cmd/roachtest/cluster.go

@@ -2382,8 +2382,7 @@ func (c *clusterImpl) MakeNodes(opts ...option.Option) string {
 }
 
 func (c *clusterImpl) IsLocal() bool {
-	// FIXME: I think radu made local more flexible and local is a prefix?
-	return c.name == "local"
+	return strings.HasPrefix(c.name, "local")
 }
 
 func (c *clusterImpl) IsSecure() bool {