dashboard/builder: pass GO386 to child builder, like GOARM
dashboard/builder: pass GO386 to child builder, like GOARM

Update Issue 7720

LGTM=dave
R=adg, dave
CC=golang-codereviews, rsc
https://codereview.appspot.com/84410046

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