dashboard/env: install dejagnu for gccgo's testsuite
dashboard/env: install dejagnu for gccgo's testsuite

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/141860043

webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools