sign_official_cos_build: Don't feed folders to evmctl

The find command feeding files to evmctl was returning directories in
addition to regular files. Add `-type f` to ensure only regular files
are handed to evmctl.

At least for me, running `evmctl ima_hash <some_folder>` yields an
`Unsupported file type (0x4000)`.

BUG=b/465211305
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I4b6d1473eae831a7ba697fb69b4c0cffffdb971c
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/vboot_reference/+/153403
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>