[secure-boot] check whether cert exists in image before duplicating work

[cjac]

script produced by shell_script_generator.py does not check whether the source image has our cert in its trust db before creating a new image for the purpose of inserting our cert.

We could avoid 3-6 minutes of many runs of generate_custom_image.py by checking whether the certs are already in the image.

I ran into this problem as I was trying to build a dask custom image based on the cuda custom image which was based on the latest 2.x-{debian,rocky,ubuntu} base images.