sweet/tile38: set GOROOT for the tile38 build

The tile38 build happens with make, so I just missed this in a previous
CL.

Change-Id: I85e06fceb2c053887afe7f04504e141e3fa27093
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/384338
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>