all: replace os.Getenv("GO_BUILDER_NAME") with testenv.Builder in tests

all: replace os.Getenv("GO_BUILDER_NAME") with testenv.Builder in tests

Some tests still reach for GO_BUILDER_NAME directly. This change makes
it so that they go through testenv.Builder.

There are a couple more, but changing them may also cause tests to start
failing. Done in a follow-up.

Change-Id: Id2453b7b62f5ebf3594e92fa53724a577a97440f
Reviewed-on: https://go-review.googlesource.com/c/go/+/703135
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>