[gpu] updates as result of rocky tests and running script with custom…

… image generator (#1242)

* [gpu] clean-up of sources.list and keyring file assertion

* allow main to access dkms certs ; remove full upgrade ; tested sources.list cleanup function

* return cert cleanup to its normal location

* merged from custom-images/examples/secure-boot/install_gpu_driver.sh

* only unhold systemd on debian12 where the build breaks otherwise