switch go-flow-levee to tagged version

this is actually a no-op source wise, but we want to prefer tagged versions

hack/tools/go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/cespare/prettybench v0.0.0-20150116022406-03b8cfe5406c
 	github.com/client9/misspell v0.3.4
 	github.com/golangci/golangci-lint v1.36.0
-	github.com/google/go-flow-levee v0.1.5-0.20210422150831-99e72693f5c8
+	github.com/google/go-flow-levee v0.1.5
 	gotest.tools v2.2.0+incompatible
 	gotest.tools/gotestsum v0.3.5
 	honnef.co/go/tools v0.0.1-2020.1.6