Replace BUILD_TIME with SOURCE_DATE_EPOCH for reproduceable builds

[ainar-g]

Currently, we compile in the date of the build. And we compile it in as plain text.

Reproducible-Builds.org recommends using the commit date instead, given through the environment variable SOURCE_DATE_EPOCH and as a Unix timestamp. This should improve the life of people building their own AGH, such as package maintainers, since this will only require a shallow clone of the repo and the “correct” Go version to build a bit-by-bit identical copy of a binary.

The documentation in scripts/README.md obviously should be updates as well.

@jamesmacwhite, @yurivict, @simonbcn, if I recall correctly, you maintain AdGuard Home packages? Do you have any objections to this?

[jamesmacwhite]

@ainar-g Thanks for this. I've contributed to the AdGuardHome package in OpenWrt, but I am not the primary maintainer of it, so I'm probably not the best person to speak for the OpenWrt package as a whole.

I'd imagine this relates to the Makefile and how they are being used when compiling in OpenWrt.

[ainar-g]

If we're doing this, we'll probably need to change the BUILD_DATE argument inside the Dockerfile as well.

Disregard that. The BUILD_DATE parameter in the Dockerfile is the actual build time for the Open Container Initiative labels.