sign_official_cos_build: make IMA signing more robust

Do not override existing ima attrs. Doing so breaks nvidia firmware
loading.

BUG=None
TEST=presubmit
RELEASE_NOTE=None

Change-Id: If2a2a804866e6a7f6b5dd373ebbdbeb12d9caef7
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/145183
Reviewed-by: He Gao <hegao@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>