Use SOURCE_DATE_EPOCH for rescue ISO timestamps

The grub-rescue-cdrom.iso file built for the Debian grub-rescue-pc package differs between builds due to timestamps on the files created inside the ISO image.

The image itself is constructed by the grub-mkrescue utility, that in turn uses xorisso, in 'mkisofs emulation' mode.

To achieve deterministic (reproducible) build output, this changeset applies two changes to the grub-mkrescue code:

• Read the timestamp to use when writing time-related information to the rescue ISO from SOURCE_DATE_EPOCH, when that variable is set (using a code snippet from the Reproducible Builds project's documentation).
• Use the --set_all_file_dates command-line option available when using xorriso -as mkisofs to apply the timestamp to all files within the image.

I've confirmed that this allows me to build the rescue ISO image deterministically on my local machine; I haven't tested the resulting image for any practical rescue purposes.

Closes: #787795