sd-image: replace faketime with --invariant for mkfs.vfat

This is done for sd-images only here, but should probably also be done
for dvd-images.

The --invariant arg should be a better way of making mkfs.vfat deterministic.
The previous version of invoking faketime was building fine and reproducible
when I was compiling an sdimage for aarch64 under emulation.
It was however still logging errors:
ERROR: ld.so: object '/nix/store/1c2cp2709kmvby8ql2n9946v7l52nn50-libfaketime-0.9.9/lib/libfaketime.so.1' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
ERROR: ld.so: object '/nix/store/1c2cp2709kmvby8ql2n9946v7l52nn50-libfaketime-0.9.9/lib/libfaketime.so.1' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
The logged errors were presumably inaccurate somehow as calling
faketime was required for reproducibility, even though the log makes it
looks like it failed.

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