cmd/coordinator: provide a GOPROXY for vetall and all subrepo builds
cmd/coordinator: provide a GOPROXY for vetall and all subrepo builds

Also allow individual builders to override GOPROXY explicitly.

The vetall builder expects to build golang.org/x/tools from a specific
revision, and in module mode that implies that it needs access to all
of the module dependencies of that revision.

The individual subrepo builders may need a GOPROXY to resolve
dependencies when the default value of GO111MODULE changes to "on",
since they will enter module mode automatically even though they are
running within GOPATH.

Updates golang/go#30228

Change-Id: I40f6f2ea3c5ed05eaaaf3503ba97e6ccd3f20e1f
Reviewed-on: https://go-review.googlesource.com/c/build/+/165738
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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