buildenv: temporarily use farmer-ui-test.golang.org as dashboard URL
buildenv: temporarily use farmer-ui-test.golang.org as dashboard URL

The build.golang.org service is undergoing a transition as part of
https://golang.org/issue/34744#issuecomment-890076711. It's taking
some time for the DNS change to propagate and HTTPS certificate to
be acquired. During this time, use farmer-ui-test.golang.org as the
dashboard URL, since it's currently used by cmd/coordinator to find
post-submit work and report post-submit build results.

For golang/go#34744.

Change-Id: I7ca99dc75afa2960f831248110c9e5053ee95f99
Reviewed-on: https://go-review.googlesource.com/c/build/+/340589
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build