main.star: stop setting GODEBUG=gotestjsonbuildtext=1 in x/build

As part of a gradual rollout, apply the change to one repo first.

For golang/go#70435.

Change-Id: I20e15cd5c72b6119696d701fe5b51b3dd6c5a417
Reviewed-on: https://go-review.googlesource.com/c/build/+/642995
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur