zip files of closures are not reproducible

[facundominguez]

Se the discussion here https://github.com/tweag/clodl/pull/36#discussion_r540817939

Related to this, we had a request to use tar as an archiving format in #31.

If tar files were easier to make reproducible, it could be a better solution than insisting on zip files.

[aherrmann]

tar and gzip have reproducibility issues as well, see here. Bazel does not include an equivalent to @bazel_tools//tools/zip:zipper for tar. However, the pkg_tar rule does produce reproducible tar files, if applicable. In case of tar and gzip it is also possible to produce reproducible archives by passing the appropriate command-line flags. See here for a wrapper script that passes these flags.