Explicitly cast seconds to NSTimeInterval in TimeIntervalFromSecondsAndNanos.

This was causing an error in our Github tests (https://github.com/protocolbuffers/protobuf/actions/runs/23308413575/job/67788944664) after Github upgraded its Macos 14 runner images. This likely seems to be due to the upgrade to Clang/LLVM, though the release notes claim the error was actually made *weaker* in 15.0.1 https://reviews.llvm.org/D133800.

Seems like https://github.com/actions/runner-images/issues/13827 is likely mistakenly causing this to surface, but this is an error in Clang16 so we should probably fix it anyways.

PiperOrigin-RevId: 886846312