build: simplify tar commands in gitian descriptors #16211

cfields mentioned that it's likely possible to simplify our use of find, sort, tar and gzip inside our gitian descriptors by taking use of modern command line flags.

We currently chain multiple commands together like so:

https://github.com/bitcoin/bitcoin/blob/7524376a81a1955f4da4adb23005f2122b18a270/contrib/gitian-descriptors/gitian-linux.yml#L152

However it should be possible to drop sort by taking advantage of tar's --sort=name command line flag:

--sort=ORDER Using --sort=name ensures the member ordering in the created archive is uniform and reproducible.

We should also be able to drop gzip and instead pass --gzip straight to tar:

-z, --gzip, --gunzip, --ungzip Filter the archive through gzip(1).

or, we may need to use -I so we can pass gzip -9n:

-I, --use-compress-program=COMMAND Filter data through COMMAND. It must accept the -d option, for decompression. The argument can contain command line options.

Is it worth it to make a single line command a bit shorter? Looks like this is working just fine, why change it?

Just for documentation, I've tried those methods and they seem to be less flexible than UNIX compose-ability with pipes. For example, the --sort=name for tar only applies to directory entries, and was a source of non-determinism in my Guix branch. Also, for --use-compress-program=, it's going to make it complicated if we need to pass options to gzip in the future that include IFS and needs escaping or something like that.

But overall, it's a good thought, and we should look for other places to optimize build time in our gitian descriptors.

The Linux piping has clear syntax and robust behavior. It uses the UNIX principle "Do One Thing and Do It Well".

Ok, closing for now, unless there is evidence that this brings any advantage.