Fails to natively decompress xz indices on macOS ("unable to download package index")

Package index fetching logic depends on the `xz` command to decompress `Packages.xz` (see https://github.com/GoogleContainerTools/rules_distroless/blob/9da4ee521ab077fbd37744cb1acdad9e0f44b349/apt/private/package_index.bzl#L8)

macOS doesn't natively have an `xz` command, the native way to decompress xz files is surprisingly `gzip -d Packages.xz` (`gzip` in macOS is a non-standard Apple variant).

It's fairly easy to workaround by installing `xz` (`brew install xz`) so the `xz` command works. Ideally rules_distroless should use the native macOS command automatically but documenting this macOS dependency in the README would be helpful too.

Start at apt/private/package_index.bzl line 8 and reproduce package index fetching on macOS. Compare the current xz-based decompression with the macOS gzip behavior described in the issue, then check the README for the relevant dependency documentation. Done means macOS users can fetch indices without an unexpected missing-command failure, or have the dependency clearly documented.

Written by the indexing model from the issue text.