[auth] Allow Milo to grab any build by its ID.
[auth] Allow Milo to grab any build by its ID.

It was previously allowed to list builders. Replace this internal
role with a new extended role (and a group) that also allows to
fetch builds by ID.

Context:
https://chromium-review.googlesource.com/c/3539405

R=968518a24f41fef00b747498600e52b792d06ae0@google.com

Change-Id: I2cb5f87e6a51695580828ffe051888466563f9cb
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3543288
Reviewed-by: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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