ui-snapshot: pass -n to gzip, to suppress timestamp

Since cgit snapshots of tags are often used for releases, we don't
want the rarely used feature of the gzip compressor that includes
an embedded timestamp into the archive, since this makes each tarball
of the same (potentially signed) tag different.

This commit refactors the archive handling code a bit so that each
different format is able to run with an arbitrary argv for the filter.