go.tools/dashboard/builder: respect CGO_ENABLED value from the environment
go.tools/dashboard/builder: respect CGO_ENABLED value from the environment

Allow builder owners to set CGO_ENABLED to override the logic in cmd/go

LGTM=minux.ma, adg
R=golang-codereviews, minux.ma, adg
CC=golang-codereviews
https://codereview.appspot.com/63570046

bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools