Make zip and tar helpers generate reproducible archives without relying on libfaketime

In the Tor Browser build process, we are using some helper scripts to generate zip and tar archives reproducibly. Those helper scripts expect the modification time of the included files to be reproducible, which is the case when we are using libfaketime during the builds.

To be able to remove the use of libfaketime in some parts of the build process and still generate reproducible archives, we could update the helper scripts to change the time of all files included in the archives according to $REFERENCE_DATETIME.