x/tools/dashboard/builder: only write VERSION file if it doesn't exist
x/tools/dashboard/builder: only write VERSION file if it doesn't exist

LGTM=dsymonds
R=dsymonds
CC=golang-codereviews
https://golang.org/cl/172610043

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