V8 Buildbot: Fix build dir for win64 trybot.
V8 Buildbot: Fix build dir for win64 trybot.

BUG=353487
LOG=n 
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/381083002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@282329 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build