sweet/tile38: set GOROOT for the tile38 build
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 <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Run-TryBot: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/benchmarks,golang/benchmarks