build_plan: allow slim only in staging for kernel 4.14 branch.

Prepare for launch by scheduling only scheduling slim builds in staging
and limiting it to the third_party/kernel 4.14 branch.

BUG=chromium:1094321, chromium:1123776
TEST=./run_tests.sh

Change-Id: Ice758dd3d68d61f846a60b48b8dc2d938410c1ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2391309
Commit-Queue: Navil Perez <navil@google.com>
Tested-by: Navil Perez <navil@google.com>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
2 files changed