[build2] Build error on openbsd 7.1

[Boris Kolpackov]

Dirty Dawn via users <users at build2.org> writes:

> Is not possible with tar/pax just received the confirmation from
> openbsd list. Suggestion was to use gtar or bsdtar.

Ok, thanks for the confirmation.

I went ahead and made these changes:

1. On OpenBSD the build system now uses OpenBSD tar without
   `--format ustar`, assuming this is the default.

2. On OpenBSD the package manager now checks if bsdtar or gtar
   can be found in PATH. If found, then it uses that and otherwise
   issues diagnostics and fails.

The today's staged toolchain contains these changes. Could you
give it a try and see if things are now working as expected?