main.star: use the test_timeout_scale property

This change sets the test_timeout_scale property instead of setting
GO_TEST_TIMEOUT_SCALE directly. This property ensures the timeout scale
propagates to all testing contexts, including `go test`, in golangbuild.

This change also allows run mods to influence the test timeout scale of
subrepo builders, since that was previously not explicitly handled, but
led to flaky test timeouts on subrepo builds.

Depends on https://crrev.com/c/5314204. This CL must wait until that
change lands and rolls out.

Fixes golang/go#65845.

Change-Id: I3d38922d64fa225da233206bf53994d4b85843c7
Reviewed-on: https://go-review.googlesource.com/c/build/+/565697
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Michael Knyszek <[email protected]>

Author: mknyszek