dashboard: set GOARCH and GOHOSTARCH explicitly for mips builders
dashboard: set GOARCH and GOHOSTARCH explicitly for mips builders

They're running the 32-bit versions on 64-bit hardware.

Change-Id: I365aa2b77cd729484d49aaefed12737c657abed8
Reviewed-on: https://go-review.googlesource.com/33010
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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