main.star: stop setting GODEBUG=gotestjsonbuildtext=1 everywhere

This is viable to do now that result_adapter is updated with better
support for Go 1.24 structured build events (CL crrev.com/c/6174231).

Fixes golang/go#70435.

Change-Id: I9afbc244438c2f8c1b0c4fddcf0bf3a04ffd6bcd
Reviewed-on: https://go-review.googlesource.com/c/build/+/642996
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur