Instead of using `libarchive` on macOS (which has a non-zero dependency
tree) let's instead use `gnu-tar` because:
- it has no dependencies
- we're already using it on Linux
I tested this locally with `ack` (which was getting reproducible bottles
with system and Homebrew `libarchive`) and saw a different checksum. A
quick read of the `gtar` manpage indicated that it uses a GNU tar format
by default which can be made consistent with BSD and GNU tar by using
`--format=ustar` with both. Once this is done, the checksums are the
same. This will hopefully ease the transition to having `all:` bottles
be consistent on Linux, too, which will be useful when homebrew-core is
fully merged/supporting Linux.
a10f436120