[milo] migrate builder.go to use global cache
[milo] migrate builder.go to use global cache

R=mwarton, nqmtuan

Bug: 1247518
Change-Id: I5ebee299dbca7e14ac6323177eae8e2e790f12b7
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3163826
Reviewed-by: Quang Minh Tuan Nguyen <7f5d2f4f6ce87649784e35ad551490ab8d89e8eb@google.com>
Commit-Queue: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go