Use the SOURCE_DATE_EPOCH timestamp [1] if it is available for
timestamps in the tarballs and the HTML documentation.
Also fix permissions inside tarballs to normalize umask
variations.
[1]: https://reproducible-builds.org/specs/source-date-epoch/
The timestamps in the .gz files are not used for anything, and they break
the reproducibility of the build. Giving --no-name means gzip will not
save the filename and timestamp in the gz file itself. When gunziping, the
current file name will be used, minus the .gz suffix.