erofs-utils: support $SOURCE_DATE_EPOCH

Currently, we use -T to set a given UNIX timestamp for all
files, yet reproducible builds [1] requires what is called
"timestamp clamping", IOW, a timestamp must be used no later
than the value of this variable.

Let's support $SOURCE_DATE_EPOCH as well.

[1] https://reproducible-builds.org/specs/source-date-epoch/

Link: https://lore.kernel.org/r/20201030123020.133084-2-hsiangkao@redhat.com
Suggested-by: nl6720 <nl6720@gmail.com>
Reviewed-by: Li Guifu <bluce.lee@aliyun.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
3 files changed