sweet: upgrade tile38 to 1.29.1

The current version of tile38 hard-codes GOFLAGS in its Makefile, which
makes experimentation with compiler flags difficult.

Upgrade to the latest version, which no longer overrides GOFLAGS. This
will probably cause a slight discontinuity in performance results.

For golang/go#55022.

Change-Id: I1e42bd76d2bfcb7b9816447b260988f37bf32c87

sweet/harnesses/tile38.go

@@ -26,7 +26,7 @@ func (h Tile38) Get(srcDir string) error {
 	return gitShallowClone(
 		srcDir,
 		"https://github.com/tidwall/tile38",
-		"1.25.3",
+		"1.29.1",
 	)
 }