Update the signing scripts to allow KMS signing in RBE.

We will pass on a flag from the builder script all the way to the signing scripts.
In the scripts - accept the new flags and pass them on to the KMS signer app.

This change must go in the same time as the BE change to add that new flag:
https://cos-internal-review.git.corp.google.com/c/cos/infra/build-executor/+/45283

BUG=b/364372578
TEST=presubmit passes on rbe (verifies impersonation) and on kokoro (verifies existing behavior when flag is not provided)

Change-Id: I544e77cdceed3c507e96699d6183118e757a9aae
cos-patch: bug
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/80899
Reviewed-by: Nandhini Rengaraj <nrengaraj@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>