Add pinned field to ReleaseBuilder proto

This will allow us to have release builders for release branches
that match the standard form release-Rxx-xxxxx.B (like LTS) which
are not at risk of being pruned as an old milestone.

BUG=b:177487002, chromium:1157948
TEST=generate.sh

Change-Id: I93c80ce9e561e4c98b62fd4399506305f85d2304
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2653505
Reviewed-by: George Engelbrecht <engeg@google.com>
Commit-Queue: Jack Neus <jackneus@google.com>
2 files changed