do not use modification time of tarball for computing %dist
Using the modification time of the snapshot tarball for computing dist is a bad idea, since it's different on different machines.
For example, the computed date during the `buildSRPMfromSCM` koji task is likely different from the local date when the package was prepared, and so package builds (especially EVRs and changelog entries) are not reproducible.
With this change, the snapshot date is not calculated magically, but the packager has to set "%global date YYYYMMDD" manually. I also adapted the documentation for the macro to reflect that change.
This is related to the following FPC issue: https://pagure.io/packaging-committee/issue/719