cmd/dist: allow $GO_TEST_TIMEOUT_SCALE to override timeoutScale

Some machines are so slow that even with the default timeoutScale,
they still timeout some tests. For example, currently some linux/arm
builders and the openbsd/arm builder are timing out the runtime
test and CL 8397 was proposed to skip some tests on openbsd/arm
to fix the build.

Instead of increasing timeoutScale or skipping tests, this CL
introduces an environment variable $GO_TEST_TIMEOUT_SCALE that
could be set to manually set a larger timeoutScale for those
machines/builders.

Fixes #10314.

Change-Id: I16c9a9eb980d6a63309e4cacd79eee2fe05769ee
Reviewed-on: https://go-review.googlesource.com/9223


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>