Support unprivileged `.pkg` installation on MacOS

[multimeric]

The regular .pkg installer seems to require admin. It should be possible to create an installation configuration that's "my user only" that doesn't require sudo, e.g. this is what Quarto now does.

e.g. this appears when I try to install https://github.com/r-lib/rig/releases/download/v0.7.1/rig-0.7.1-macOS-arm64.pkg: