[milo] Fix builder pool
[milo] Fix builder pool

The code was refactored from milo's BuilderID to buildbucket's BuilderID
and they have different String() implementation. Use the right one.

R=tandrii@chromium.org

Bug: 980734
Change-Id: Iebd3dc12639b8d74fa86db56347fde2a8bee0414
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1688509
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>

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