uefi: os-prober doesn't skip cdrom iso9660 type

File: /usr/lib/os-probes/50mounted-tests

Issue: on a respin of Ubuntu (yes, not supported but I believe the issue could possibly affect Ubuntu as well) when booting the USB in 'secure boot' UEFI mode, when installing, the update-grub command gives the following error which then stops the installer with the all-too-common "failed to install grub-efi-amd64-signed" to /target error:

grub-probe: error: cannot find a GRUB drive for /dev/sdb1. Check your device.map.

The problem is, however, that in this case /dev/sdb1 is the live system's "cdrom" and should NOT be probed! So I propose the following addition to 50mounted-tests that would ensure "optical media" partitions would be skipped by os-prober:

elif [ "$types" = iso9660 ]; then
    debug "$1 is an optical media iso9660 partition; skipping"
    exit 0

With this addition, update-grub no longer returns an error which I am hoping will make ubiquity not give the following error:

ubiquity: installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 127
ubiquity: Errors were encountered while processing:
ubiquity: grub-efi-amd64-signed
ubiquity: E: Sub-process /usr/bin/dpkg returned an error code (1)

I hope this addition will help others that have issues with secure boot mode! I have not created a patch yet but can if it seems acceptable.