cloudbots: use proxy when reconnect for B120

BUG=b:369379070

Change-Id: I16669b024c7b374843f3ebb4cd1866d4698ddda9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/5888650
Tested-by: Bryan Ni <bniche@google.com>
Reviewed-by: Azizur Rahman <azrahman@google.com>
Reviewed-by: Garry Wang <xianuowang@chromium.org>
Commit-Queue: Bryan Ni <bniche@google.com>
Auto-Submit: Bryan Ni <bniche@google.com>