Omit timestamps and names from gzip header

The gzip header contains optional slots for the last-modified time and the filename. In service of the ideal of fully-reproducible builds, these should be omitted. Some effort has gone into this already, involving the normalization of timestamps inside the tar archive. However, while the use of pipes eliminates the filename field from the .tar.gz file, it causes the modification time to be set to the time of execution; use of the -n flag turns off both these fields.

(Interestingly, it appears that the signature prepended to the control.tar.gz file by abuild-sign already omits these fields.)