Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

It changes the following:

• checks for SOURCE_DATE_EPOCH env and uses it as build_date

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

Allow to override build date with SOURCE_DATE_EPOCH
to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This patch was done while working on reproducible builds for openSUSE.

[extrawurst]

thanks for the contribution, just a very small nitpick, otherwise good to go

[bmwiedemann]

Are more changes needed?

[extrawurst]

Can you fix the CI issues?

[extrawurst]

@bmwiedemann it really is nothing more than running cargo fmt so that it formats the build.rs file you editted

[extrawurst]

@bmwiedemann did these changes and merged it, thanks for your contribution