UEFI_COMPATIBLE must be set in order to use image_signatures_db

[rogerhu]

Overview of the Issue

In order to set image_signatures_db, the UEFI_COMPATIBLE flag must be passed as a guest OS feature:

source "googlecompute" "gcp_re_debian12_x86_64" {
   arch = "x86_64"
  
  image_guest_os_features = ["UEFI_COMPATIBLE"]
  image_signatures_db     = local.image_signatures_db
}

However, when using VMs that inherit from others, specify the image_guest_os_features would overwrite the ones that get used. The gcloud SDK actually doesn't check this guest OS features (https://github.com/google-cloud-sdk-unofficial/google-cloud-sdk/blob/master/lib/surface/compute/images/create.py#L308-L321) and allows the image signatures to be passed in regardless.

Reproduction Steps

Don't set image_guest_os_features = ["UEFI_COMPATIBLE"] when setting image_signatures_db

Plugin and Packer version

Packer 1.2.5 verison

Operating system and Environment details

Linux x86 and arm64