go.tools/dashboard/builder: try to recreate workpath if already exists
go.tools/dashboard/builder: try to recreate workpath if already exists

Fixes golang/go#8714.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/148010043

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