archives: update paragraph about `tar` and `--clamp-mtime` flag

$ zip -r product.zip build

{% endhighlight %}

A patch has been written to simplify the latter operation with GNU

Tar. It is currently available in Debian since

[tar](https://packages.qa.debian.org/tar) version 1.28-1. Hopefully it

will be integrated upstream soon, but you might want to use it with

caution. It adds a new `--clamp-mtime` flag which will only set the time

In `tar` >= 1.29, the `--clamp-mtime` flag can be used to only set the

when the file is more recent than the value specified with `--mtime`:

{% highlight sh %}

# Only in Debian unstable for now

$ tar --mtime='2015-10-21 00:00Z' --clamp-mtime -cf product.tar build

{% endhighlight %}