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/+/39983

BUG=b/332363254
TEST=presubmit passes on rbe (verifies impersonation) and on kokoro (verifies existing behavior when flag is not provided)

Change-Id: I544e77cdceed3c507e96699d6183118e757a9aae
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/67690
Tested-by: RBE Service account <service-384042960741@remotebuildexecution.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>