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

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/141860043

evandbrown/build,golang/build,proppy/build,golang/build,evandbrown/build,proppy/build,evandbrown/build,proppy/build,golang/build,golang/build,proppy/build