go.tools/dashboard/builder: use $home on Plan 9
go.tools/dashboard/builder: use $home on Plan 9

LGTM=alex.brainman
R=golang-codereviews, lucio.dere, alex.brainman
CC=golang-codereviews
https://golang.org/cl/64520043

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