[realms] Add resultdb.reader to buildbucket.builderServiceAccount.
[realms] Add resultdb.reader to buildbucket.builderServiceAccount.

It would allow builders to read and include invocations created
by subtasks (e.g. isolated tests) running in the builder's realm.

This is mostly needed for non-public builders.

R=332b6c56634d0f63001046c664a2370006a02420@chromium.org
BUG=1193458

Change-Id: I8a20b1ae6c2c778fb1213e44b3c4e14da81806d1
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2800146
Auto-Submit: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Commit-Queue: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py