* Do not run `tune2fs` on the image. The file system is not mounted, so
the default values for "Maximum mount count" (`tune2fs` option `-c`) and
"Check interval" (`tune2fs` option `-i`) are fine as is. With this change
the ext4 image is reproducible.
* Remove `E2FSPROGS_FAKE_TIME`. e2fsprogs 1.47.1 supports
`SOURCE_DATE_EPOCH` so setting this e2fsprogs specific variable is not needed
anymore.
See https://github.com/tytso/e2fsprogs/issues/131 for details.