Respect SOURCE_DATE_EPOCH when building

[toabctl]

To make the build reproducable (see [0]), respect the SOURCE_DATE_EPOCH variable and don't include a build timestammp in the binary when set.

[0] https://reproducible-builds.org/

[toabctl]

tested with:

❯ make dist     
CGO_ENABLED=0 go build -o listmonk -ldflags="-s -w -X 'main.buildString=v5.1.0 (#f8dee184 2025-12-05T07:48:45+0000)' -X 'main.versionString=v5.1.0'" cmd/*.go

vs.

❯ SOURCE_DATE_EPOCH=1 make dist 
CGO_ENABLED=0 go build -o listmonk -ldflags="-s -w -X 'main.buildString=v5.1.0 (#f8dee184)' -X 'main.versionString=v5.1.0'" cmd/*.go

[knadh]

Hi @toabctl. If SOURCE_DATE_EPOCH is set, it should be used as the Unix timestamp offset and the date should be derived from that, otherwise, the system time. Here, if it's set, the date bit in the build string is completely eliminated, which isn't right.