`tar` on OpenBSD

[rgrinberg]

On OpenBSD, opam uses gtar rather than tar. We should do the same, or figure out how to use tar from OpenBSD.

[shonfeder]

Could we address this using #10127 ?

[mndrix]

Related to tar on OpenBSD. When using dune pkg lock I get an error message because OpenBSD's tar needs an explicit -z in order to decompress gzipped input:

File "dune.lock/ocaml-compiler.5.4.0.pkg", line 407, characters 3-76:
407 |    https://github.com/ocaml/ocaml/releases/download/5.4.0/ocaml-5.4.0.tar.gz)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: failed to extract 'download'
Reason: 'tar' failed with non-zero exit code '1' and output:
- tar: input compressed with gzip; use the -z option to decompress it