Add sbom tag for cos-gpu-installer

For SBOMs generation we need to apply sbom, sbom_arm64 and sbom_amd64
tags to container images. The recent changes(go/cos-rev/96659) forgot
to attach sbom tag and this is the fix.

BUG=b/397500013
TEST=presubmit

Change-Id: I69b8f4f9fd9a3c1aedce2b42fae805fb27f62f68
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/96841
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>
Tested-by: Chenglong Tang <chenglongtang@google.com>