Allow overriding -mkfs-time with SOURCE_DATE_EPOCH

[lexi-the-cute]

I'm trying to make my builds reproducible with all platforms. Unfortunately mksquashfs bails if both time args such as -mkfs-time is set and the environment variable, SOURCE_DATE_EPOCH, is set.

I'd like to be able to unset the -mkfs-time option if the environment variable is set.