Improve reproducibility

1. When using a (partial) mirror without the task packages, perl will complain a lot about this unset variable.

2. gzip will embed the timestamp of the file when compressing, avoid that. (In my opinion it would be an even better to remove the work around for a bug in busybox that has been fixed since >10 years).

3. mkfs.msdos will embed random serial numbers, pass the same options --invariant -i deb00001 as used in debian-installer: https://salsa.debian.org/installer-team/debian-installer/-/blob/985e0853cbae10d7ee1c4020d6330c80f0695a5b/build/util/efi-image#L200

NOTES:

• I have not tested these changes in this exact form, they are applied in a container build using some sed. I then use simple-cdd to build my images.

• There are some further issues with modified timestamps related to isolinux that I've not been able to track down, inserting calls to a clamp_mtimes function before each invocation of make in simple-cdd fixes that mostly

• The make_image tool also touches a few files, modifying the timestamps, i.e. just before packing the image at https://salsa.debian.org/images-team/debian-cd/-/blob/0dd4219af48f2ffd9d86d6e79a71af4c9b813cf0/tools/make_image#L147-148