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
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/444475
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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",
  )
 }